Validating Threat: IO Approval and Public Support for Joining Military Counterterrorism Coalitions

Recent scholarship has fruitfully investigated the effect of international organization (IO) approval on public support for military intervention. Following Jentleson and Britton [Bruce W. Jentleson and Rebecca L. Britton, “Still Pretty Prudent: Post-Cold War American Public Opinion on the Use of Military Force,” Journal of Conﬂict Resolution 42, no. 4 (1998): 395–417], scholars argue that IO approval does not increase already high public support for “foreign policy restraint” (FPR) operations intended to coerce “aggressively threatening” opponents, including terrorists. We challenge this argument, focusing on pub- lic support for contributing to military coalitions. The public may wonder whether leaders are sincere when they frame a coalition military operation as having FPR objectives; this may lead the public to put a premium on multilateral validation. We also question the common argument that UN Security Council approval necessarily has a greater positive effect on public support for intervention than approval from regional IOs. Approval from broad-based regional IOs, such as the African Union (AU), may be just as consequential. Data from survey experiments that we conducted in three countries conﬁrm our principal hypotheses: (1) IO approval consistently increases public support for contributing to military coalitions even in counterter- rorism cases and (2) the UN and AU approval effects are of comparable magnitude. These ﬁndings expand our theoretical understanding of the conditions under which IO approval can increase public support for military intervention.


Introduction
In January 2013, France sent more than 4,000 combat troops to Mali to repel Islamist terrorists affiliated with Al Qaeda who were poised to capture the capital, Bamako. In response to France's pleas for assistance, the United States, Canada, and fifteen European countries joined the effort, providing aerial lift, refueling, and reconnaissance support, and by April, a coalition of 6,000 West African troops had deployed alongside French forces (Chivvis 2016, 115-41). More than a year later, in September 2014, the United States launched a military counterterrorism operation in the Middle East consisting of sustained air strikes against the Islamic State in Iraq and Syria (ISIS). US policymakers, too, worked hard to recruit international partners, and ultimately more than twenty countries made significant contributions to the anti-ISIS coalition, bombing terrorist strongholds and training local counterterrorism forces (Saideman 2016). These instances show that military interveners generally value multinational coalition support, because it helps them share op-Stefano Recchia holds the John G. Tower Chair in International Politics as an Associate Professor at Southern Methodist University. His research interests revolve around the politics and ethics of military intervention. Further information about his work is available at www.stefanorecchia.net. Jonathan A. Chu is a Lecturer at Stanford University. He specializes in public and elite opinion on issues of democracy, governance, and foreign policy. Further information about his research is available at www.jonathanchu.org.
Author's note: Both authors contributed equally to this article. The authors' names alternate between first and second position across several papers that are part of a broader ongoing research project. The authors are grateful to Terry Chapman, Atsushi Tago, the anonymous reviewers, and the ISQ editors for valuable comments that helped significantly improve this article. The authors would also like to thank Dr Joel Rogers de Waal, research director at YouGov UK, for generously supporting this project. The data underlying this article are available on the ISQ Dataverse at https://dataverse.harvard.edu/dataverse/isq. erational risks and burdens and may increase the legitimacy of their actions (Kreps 2011;Weitsman 2014;Recchia 2015).
A well-established literature argues that countries, especially liberal democracies, can increase support for military intervention among their domestic public by securing approval from international organizations (IOs) such as the United Nations and various regional bodies (Schultz 2003;Chapman and Reiter 2004;Fang 2008;Grieco et al. 2011). Building on these arguments, scholars have hypothesized that IO approval might also raise support for intervention among foreign publics by transmitting reassuring information about the policy's merits and appropriateness, thus making it easier for partner states to contribute to military coalitions (Thompson 2006(Thompson , 2009Tago 2007;Chapman 2011, chapter 5;Tago and Ikeda 2015).
We bring this "information transmission" literature on IO approval into dialogue with a different strand of scholarship, which argues that multilateral approval does not increase already high public support for "foreign policy restraint" (FPR) military operations that aim to coerce aggressive and threatening opponents (Jentleson and Britton 1998)including counterterrorism operations (Eichenberg 2005; see also Gelpi, Feaver, and Reifler 2009). We zero in on public support for contributing to coalition-based counterterrorism operations, probably the most frequent type of FPR mission in recent years.
The public, we note, may wonder whether leaders are sincere when they frame a military operation as "counterterrorist"-all the more when the leaders in question are from a foreign country intent on garnering coalition support. Leaders have a tendency to inflate security threats to sell military intervention, which is something that the public may be aware of or at least suspect (Kaufmann 2004;Mueller 2006). More generally, international threats, including those from militant groups abroad, are often indeterminate, which may leave the public craving a "second opinion." We therefore hypothesize that the public may put a premium on multilateral validation through impartial IOs, even in prima facie least likely FPR cases such as counterterrorism.
In addition, we take a closer look at the common argument that United Nations Security Council (UNSC) approval for the use of force has a stronger legitimation effect, and thus a greater positive impact on public support, than approval from regional IOs (see, e.g., Risse-Kappen 1997;Ku and Jacobson 2003;Thompson 2006Thompson , 2009Tago 2007;Westra 2007;Chapman 2011). To date, the UNSC's stronger legitimation effect has largely been assumed, and to our knowledge, there have been no studies testing the argument using causally identified evidence. We challenge this argument and develop and test an alternative hypothesisnamely, that regional IOs with broad and diverse membership, especially continent-wide bodies such as the African Union (AU), may be no less effective than the UNSC at increasing public support for intervention.
To assess these hypotheses, we fielded nationally representative surveys in three countries-the United States, Great Britain, and Germany-asking about public willingness to contribute to a French-led intervention in Africa to fight Islamist terrorists, with an embedded experiment about UNSC and AU approval. Survey experiments have the advantage of yielding unbiased estimates of causal effects compared with traditional observational studies, because the randomization of stimuli makes the treatment and control groups equal, on average, in terms of observed and unobserved characteristics, while also removing concerns about reverse causality. Experimental studies therefore strengthen the internal validity of social science research. It follows that, even to the extent that an experimental study merely replicates a previous observational study, it still adds obvious value, because it can solidify (or not) the causal nature of a previously suggested relationship (Mutz 2011, 157). Furthermore, experiments that use nationally representative samples, like the present one, have clear advantages in terms of external validity-that is, the generalizability of findings to the general population-compared with laboratory experiments involving smaller/nonrepresentative samples.
The results of our study confirm our principal hypotheses. First, IO approval consistently increases public support for contributing to military coalitions, even in ostensible counterterrorism cases. The effect of IO approval is notable across all three countries; we find that support increases for contributing combat troops, finance and logistics, and expert advisers. This solidifies the causal nature of information transmission arguments, which claim that IO approval can send reassuring information about an intervention's appropriateness. At the same time, our results challenge the argument that multilateral approval has no discernible effect on public support for FPR missions (Jentleson and Britton 1998;Eichenberg 2005). Second, we find that the effects of UNSC and AU approval on public support are of comparable magnitude. This confirms the hypothesis that broad-based, continental regional IOs, which are likely to be independent of particular interests, can be just as effective as the UNSC in terms of reassuring foreign publics and facilitating multinational coalition building. These new findings expand our theoretical understanding of the conditions under which IOs can rally international support for military interventions.
The rest of the article is structured as follows: The first part fleshes out the theoretical argument underlying the null hypothesis-that IO approval has no positive effect on public support for contributing to FPR missions such as counterterrorism. The second part develops our main alternative hypothesis: in light of the indeterminate nature of international threats and the possibility of threat inflation by political leaders, IO approval may signal to foreign publics that the threat is real and the intervention policy is well considered, thereby facilitating multinational coalition building. The third part lays out the common argument that UNSC approval inherently has a greater positive effect on public support than endorsements from regional IOs, considers why this argument may be problematic, and develops our alternative hypothesis that broad-based regional IOs may be able to match the UNSC's information-transmission capabilities. The fourth part describes our research design, presents our findings, and discusses their implications. The fifth part extends our investigation with an ancillary analysis of how IO approval interacts with the political party affiliation of individual respondents.

Is IO Approval Unnecessary for FPR Operations?
Countries leading military interventions are likely to find it easier to recruit international coalition partners if they can convince not only foreign leaders but also foreign publics that the policy is worth supporting. While foreign leaders may be sympathetic to an intervention for strategic reasons, they may nevertheless refrain from contributing to a coalition if their domestic public is skeptical of becoming involved. For example, in the case of the US-led Iraq invasion of 2003, negative public opinion across many countries restricted international troop contributions (Goldsmith and Horiuchi 2012).
To persuade foreign publics to contribute to a military coalition, the lead intervener may highlight the goal of neutralizing a shared threat. Walt (1987) famously theorized that shared threat-determined by, among other things, the power and perceived offensive motives of a common opponent-facilitates international alliance formation. Building on this, scholars have argued that shared threat also makes it easier for states to join ad hoc military coalitions and "incur the costs, [in terms of] life lost and money spent-of providing military support" (Davidson 2011, 16; see also Bennett, Lepgold, and Unger 1994, 43). Public opinion research confirms that democratic audiences are more supportive of military intervention when they perceive that an aggressive opponent threatens important security and economic interests (Russett and Nincic 1976;Herrmann, Tetlock, and Visser 1999;Everts and Isernia 2015, 98-108). Thus, the more a common threat is readily apparent and compelling, the more the lead intervener will be able to effectively sell to international partners a policy intended to tackle the threat.
Transnational terrorism has ranked as one of the principal security threats over the last two decades among publics in the United States as well as in Europe-including Great Britain, Germany, and France (Everts and Isernia 2015, 69-78). Islamist terrorism, in particular, has been framed as a vivid security threat to Western democracies, and there is evidence that the availability of such "strong negative images will make [military] interventions more compelling" (Brownlee 2020, 267). This suggests that perceptions of shared threat from transnational terrorism may generate strong public support for military intervention and for contributing to related multinational coalitions. If support for joining coalitions to fight terrorism is naturally strong, then further legitimation through multilateral approval (e.g., from the UNSC) should be only marginally effective or outright unnecessary.
The hypothesis that IO approval does not meaningfully increase public support for joining military counterterrorism coalitions is consistent with a prominent strand of public opinion research, which starts from the observation that baseline public support for armed intervention is a function of the "principal policy objective." Support will be highest for FPR interventions aimed at coercing an adversary that "aggressively threatens" the national homeland, its citizens, or key allies, and lowest for interventions seeking internal political change (IPC) in the absence of major security threats (Jentleson 1992, 53-54; see also Oneal, Lian, and Joyner 1996). Expanding on this research, Jentleson and Britton (1998, 406) found that IO approval does not further raise public support for FPR missions (although it does so for IPC missions). The main takeaway is that when the public readily supports an armed intervention, as in the case of FPR missions, IO approval is unlikely to make a notable difference to that support.
Counterterrorist military action aims to neutralize ostensibly threatening adversaries, and it typically elicits high levels of public support; this has led scholars to conclude that "combating terrorism should be identified as a new FPR mission" (Gelpi, Feaver, and Reifler 2009, 73; see also Eichenberg 2005, 157). If one accepts this categorization, military counterterrorism operations would appear to have become the most frequent type of FPR mission over the last two decades, eclipsing traditional operations to repel interstate aggression. Eichenberg (2005, 160) specifically investigated the effect of multilateral endorsements on public support for military counterterrorism operations: examining aggregate polling data from the post-9/11 period, he found that, as for FPR missions more generally, multilateral endorsements had no significant impact.
High threat perception increases belief in the urgency and necessity of military action: people feel the need to neutralize aggressive opponents intent on lashing out at their community or their country. Meanwhile, securing IO approval from the UNSC or regional bodies often takes a long time; it limits interveners' freedom of action; and it may require difficult compromises in terms of operational plans and targeting that risk compromising military effectiveness. 1 Securing IO approval, in short, can be costly. This may explain why, when faced with short-term challenges to national security, states often choose to bypass multilateral bodies (Kreps 2011). People asked whether they support contributing to a military coalition to fight Islamist terrorism may be aware of the costs of securing IO approval and may deem these costs exceedingly high; consequently, they may agree to support the military effort even without IO approval. Indeed, it is conceivable that to maximize freedom of action in the fight against terrorism, the public might prefer to bypass relevant IOs altogether.

How Compelling Is the Threat? The Payoffs of IO Validation
Radical Islamist organizations in the Global South, such as those fought by France and its partners in Mali and the broader Sahel region of West Africa, constitute a formidable threat to local populations. Yet it is unclear whether, and if so to what extent, such organizations threaten the security of people in the industrialized democracies of the Global North, including Europe and the United States. Militant organizations in sub-Saharan Africa may lash out rhetorically against countries in the Global North, but their violence has primarily targeted fellow Africans. Indeed, these organizations are often rooted as much in local ethnic resistance movements as in global Islamic jihad (ICG 2019). The indeterminate nature of foreign security threats, including those from putative terrorist havens abroad, and the complexity of international politics more generally make it difficult for average citizens to assess the likely benefits and costs of military intervention. Sometimes external threats are truly compelling (as in the face of a direct, large-scale attack against fellow citizens or close allies), which may generate broad public support for military intervention. 3 More often than not, however, when democratic leaders argue in favor of military action abroad, domestic audiences face an asymmetric information problem: they know little about the precise nature of the threat and likely will struggle to determine whether the proposed course of action is appropriate. Political leaders might exploit this information asymmetry, inflating foreign security threats and using the terrorism label strategically-precisely to build up public support for military intervention (Kaufmann 2004;Mueller 2006).
Suspicion that leaders have inflated a foreign threatfor example, by branding an insurgent group abroad as "terrorists"-may leave domestic audiences skeptical or disoriented. In such circumstances, IO approval could serve as a useful informational device, especially for "rationally ignorant" members of the public who do not devote much attention to foreign affairs. It could reassure them of the appropriateness of military intervention and its likely beneficial consequences in terms of neutralizing, or at least reducing, a genuine threat. In other words, IO approval might offer an impartial certification, or "second opinion," to domestic audiences that a foreign actor-whether a third country or a militant organization-indeed constitutes a serious threat and that military intervention to counter the threat is well considered and legitimate (Fang 2008;Chapman 2011;Grieco et al. 2011;Chu 2019).
One would expect the payoffs of IO validation to be all the more apparent when the lead country in an intervention seeks to persuade foreign publics to contribute to a multinational coalition. Foreign publics may be especially reluctant to support their government's contribution to a military operation launched by another state about whose preferences and objectives they know little, in the absence of IO validation. As Thompson (2006, 12) puts it, "a coercer [i.e., principal intervener] that achieves IO approval makes it easier for foreign leaders to offer support"; specifically, IO approval facilitates coalition contributions "by minimizing domestic opposition" (see also Chapman 2011, chapter 5). This leads us to formulate the following hypothesis with specific regard to military counterterrorism coalitions.

Hypothesis 3 (Multilateralism boost): If the lead state in a military counterterrorism operation secures IO approval, that increases domestic support among potential coalition partners for contributing to the effort.
The argument that IO approval increases public support for contributing to multinational coalitions is logically compelling and has significantly improved our understanding of the politics of modern-day coalition warfare. However, there are several aspects of the argument that have yet to be fully tested. Thompson (2006Thompson ( , 2009 is primarily interested in theory development and provides only initial evidence from case studies of US-led operations against Iraq. Two further studies by Tago (2007) and Chapman (2011, chapter 5), which draw on large-N statistical analysis, offer the important finding that military coalitions are more frequent when the lead intervener enjoys UNSC approval. But these studies do not explicitly test for the causal effect of IO approval on public support for joining such coalitions. Furthermore, the authors' analysis may be frustrated by selection bias: military interventions since the end of the Cold War have been much more likely to enjoy both IO approval and significant coalition contributions, yet this does not necessarily prove that the former causally influenced the latter.
To our knowledge, the only survey experimental tests of the IO approval effect on foreign public support for contributing to military coalitions have been performed by Maki Ikeda and Atsushi Tago (Ikeda and Tago 2014;Tago and Ikeda 2015). 4 The authors' most important finding for our purposes is that successful UNSC approval for hypothetical US-led interventions makes Japanese respondents more likely to support the provision by their government of logistical military assistance (Tago and Ikeda 2015). The authors, however, do not investigate public support for FPR interventions aimed at coercing aggressive opponents. (Their closest scenario involves naval patrol operations against piracy in the Middle East; yet piracy in that region is unlikely to be perceived as a major national security threat to Japan, making low baseline public support and a notable IO approval effect quite likely a priori.) Furthermore, Tago and Ikeda focus on the UNSC, without examining endorsement from regional IOs. Perhaps most importantly, because of the unusual pacifism of modern-day Japanese citizens, it is unclear whether the authors' finding of a significant UN approval effect is generalizable to other countries. Tago and Ikeda (2015, 399) acknowledge that "it would be ideal" to conduct additional survey experiments involving different countries, "to examine possible cross-national differences." In addition, the existing survey experimental literature leaves unanswered how IO approval affects public support for different types of military coalition contributions. Such contributions can range from deploying combat troops, to offering financial and logistical assistance, to merely sending experts. As noted, Tago and Ikeda (2015) focus on Japan's potential provision of logistical assistance to a US-led military coalition. They do not investigate support for offering combat troops or other types of assistance. This might reflect Japan's constitutional restrictions on the use of military force or perhaps the expectation that general pacifist attitudes would largely suppress Japanese support for contributing combat troops. However, this leaves an important question unanswered: Could the effect of IO approval on public support depend on the type of coalition contribution being considered? Specifically, could there be a direct relationship between the size of the IO approval effect and public perceptions that the particular contribution envisioned involves significant risks? Previous research has found that democratic citizenries are generally quite prudent, or risk-averse, when it comes to military intervention. The public is more likely to support military action when it believes there is a reasonable chance of success; it also generally displays a preference for minimizing related costs and commitments (Jentleson and Britton 1998;Eichenberg 2005). This same research found a sizeable IO approval effect on public support for higherrisk IPC missions, in which success often remains elusive and open-ended commitments can be expected (Jentleson and Britton 1998, 406). Asked to support contributions to military coalitions, the public might attach greater value to IO approval and the attendant impartial confirmation that the policy is worth pursuing, when the contribution under consideration appears particularly risky. Thus, public support for contribution policies that involve the greatest risk should benefit most from receiving IO approval. This leads us to formulate the following hypothesis.

Hypothesis 4 (Policy risk):
The IO approval effect on public support will be greater for riskier coalition contributions, such as deploying combat troops, and smaller for less risky contributions, such as sending experts and offering financial and logistical assistance.

UNSC versus Regional Approval
Many scholars argue that UNSC approval has a stronger legitimation effect on military intervention than approval from regional IOs, and thus a greater positive impact on public support. Explanations emphasize that UNSC approval carries unparalleled authority as a symbol of international appropriateness (Risse-Kappen 1997;Finnemore 2003;Hurd 2007); that public audiences view the legality stemming from UNSC approval as a default test of legitimacy (Ku and Jacobson 2003;Westra 2007); and that the UNSC's greater impartiality and heterogeneity of membership are better able to reassure skeptical publics (Thompson 2006;Chapman 2011). 5 From an information-transmission perspective, an IO's ability to reassure skeptical publics about a proposed intervention-and, in the case of foreign publics, persuade them to contribute to multinational coalitions-is a function of the organization's independence from particular interests and the distance of its aggregate preferences from those of the principal intervener. These factors, in turn, are likely related to the breadth and diversity of the organization's membership. According to Thompson (2006, 7), "it is the neutrality of IOs"-their independence from particular interests-that allows them to serve as informative agents of the international community. Regional IOs, Thompson (2009, 38) affirms, "are less neutral" than the UNSC; consequently, "while some information will be transmitted to third parties (i.e., potential coalition partners) if coercion is channeled through regional organizations, this choice of forum is less effective" for reassuring foreign audiences and persuading them to contribute. Chapman (2011, 132) reaches the same conclusion from a somewhat different premise: because the UNSC is "a more conservative source" that is unlikely to authorize uses of force that would disrupt the status quo, approval from this body is better able to reassure foreign publics and stimulate coalition contributions than approval from regional IOs.
Some scholars go so far as to claim that regional IOs, given their less than universal nature, can legitimize military intervention and increase public support only among their own member state audiences (Tago 2007; see also Slater 1969). From this perspective, if the lead state in an intervention secures AU approval, for example, it is likely to be able to raise domestic support for coalition contributions among African audiences, but not among publics elsewhere. These arguments can readily be applied to counterterrorism coalitions. Hypothesis 5 (UNSC bonus): UNSC approval increases public support for contributing to military counterterrorism coalitions more than approval from regional IOs.
Hypothesis 6 (Regional IOs and non-members): Regional IOs cannot increase public support for joining military counterterrorism coalitions among non-member states.
The foregoing arguments do not explicitly differentiate among types of regional IOs. Yet all regional IOs may not be created equal. Recchia (2020a) suggests that subregional IOs with fairly homogeneous membership and limited independence from powerful states, such as the Caribbean Community (CARICOM) and the Economic Community of West African States (ECOWAS), can indeed be expected to offer only scant value as tools to reassure foreign audiences about an intervention's appropriateness. By contrast, he argues, continent-wide regional IOs, such as the AU and the Organization of American States (OAS), usually have broad and diverse membership and are quite independent of particular interests; this makes such regional IOs similar to the UNSC in terms of their information-transmission benefits. If follows that approval from continent-wide regional IOs may not be less valuable than UNSC approval for legitimizing military intervention even beyond the organization's own members. Indeed, when such broad-based regional IOs encompass the territory targeted by a particular intervention (e.g., the AU for interventions in Africa), they might have an advantage over the UNSC, insofar as they can signal a politically valuable form of local consent, undermining arguments about "neo-imperialism." Hypothesis 7 (UNSC null): UNSC approval does not increase public support for contributing to military counterterrorism coalitions more than approval from continent-wide regional IOs.

Empirical Context
In our empirical investigation, we examine how IO approval affects US, British, and German public support for contributing either combat troops, finance and logistics, or expert advisers and trainers to a prospective French-led counterterrorism intervention in West Africa. France is one of the world's most interventionist countries, having launched more than a dozen major combat operations in Africa since the end of the Cold War, in countries such as the Central African Republic, Chad, Côte d'Ivoire, and Mali. Importantly, these operations have relied quite heavily on multinational coalition contributions, and French leaders have increasingly sought IO approval to facilitate coalition building (Bourmaud 2011;Chafer, Cumming, and Van der Velde 2020). Several of these operations have been framed as targeting Islamist terrorists (Pannier and Schmitt 2019). The United States, Great Britain, and Germany have been among France's preferred coalition partners over the last two decades, contributing combat troops, logistical support, and trainers (Tardy 2020; see also Chivvis 2016; Gegout 2018). Consequently, our survey scenarios are realistic as well as politically salient.
Our choice of empirical context is also important for scholarship. The majority of studies on IO approval and coalition warfare to date have focused on operations led by the United States (e.g., Thompson 2009;Davidson 2011;Kreps 2011;Weitsman 2014;Recchia 2015;Schmitt 2018). Thus, we do not know whether the evidence for existing theories is generalizable beyond the US case. In fact, it can be argued that the United States is an "easy case" to test theories of IO legitimation. The United States, as the world's preeminent power, is often accused of harboring imperial designs over foreign countries. The 2003 US invasion of Iraq, widely viewed as illegitimate among foreign audiences, appears to have further increased international skepticism about the rationales for intervention proffered by US leaders (Ruggie 2007;Everts and Isernia 2015). This makes a significant IO approval effect on foreign public support for joining US-led coalitions highly likely a priori. By contrast, interventions led by second-tier powers, such as France, are sometimes controversial in the target region, but with few exceptions, they are unlikely to elicit the same degree of international scrutiny and controversy (Recchia 2020b, 513-14). A significant IO approval effect for French interventions is thus less likely a priori and more revelatory if discovered.
Before proceeding to the empirical analysis, we would like to clarify one additional point. We do not claim that securing IO approval is, in and of itself, sufficient for military interveners to forge multinational coalitions. Recent studies have usefully highlighted the importance of political and economic side payments as a tool for military coalition building (Kreps 2011, 10;Wolford 2015;Henke 2019a). Countries with wide-ranging diplomatic networks and extensive bilateral and multilateral institutional ties, such as the United States and France, can leverage their "diplomatic embeddedness" to channel side payments more effectively, thus greatly facilitating multinational coalition building (Henke 2019b). 6 Nevertheless, even for such diplomatically embedded countries with significant international leverage, IO approval may substantially facilitate multinational coalition building by increasing foreign public support. Side payments and other material incentives may be insufficient to overcome foreign publics' resistance to joining multinational coalitions-as evidenced by the reluctance of many countries to join the initial phase of the 2003 Iraq invasion, given the absence of IO legitimation, in spite of significant US pressure and side payments (Ruggie 2007;Goldsmith and Horiuchi 2012).

Research Design
To assess the effect of IO approval on foreign public support for contributing to military counterterrorism coalitions, we conducted three nationally representative opinion polls in the United States (N = 1,226), Great Britain (N = 1,697), and Germany (N = 1,129). The surveys were fielded online by the polling company YouGov in August 2018, in each country's national language. YouGov has a track record of outperforming other survey platforms in accuracy (Twyman 2008;Rivers 2016). The company uses matching techniques to achieve nationally representative samples; specifically, it recruits participants to join standing survey panels, from which it samples survey takers in such a way to meet benchmarks that are representative of the national adult population. 7 In our study, each survey taker read a short vignette that began with the introduction, "Imagine that France is planning a military intervention in one of its former colonies in West Africa." 8 Then, the vignette described France's intervention goal as helping the African country's government combat Islamist terrorists. Within each vignette, an experiment was embedded in which we randomly assigned information about whether the UNSC and/or the AU endorsed France's intervention. This created four experimental conditions: 1. "Neither the UNSC nor the African Union has endorsed the intervention." 2. "The UNSC, but not the African Union, has endorsed the intervention." 3. "The African Union, but not the UNSC, has endorsed the intervention." 4. "Both the UNSC and the African Union have endorsed the intervention." Lastly, the respondents were asked to express their support for their national government (i.e., the United States, Great Britain, or Germany) contributing to France's intervention in three possible ways: by deploying ground troops to help and potentially fight alongside French forces; providing financial and logistical assistance; and sending experts to advise and train local government forces. The response choices were: strongly support, somewhat support, somewhat oppose, strongly oppose, and do not know. Ideally, we would have added further experimental conditions to compare and contrast the effect on foreign public support of approval from various other regional IOs besides the AU-including, for example, the European Union, NATO, ECOWAS, and the Arab League-but budgetary and survey sample size constraints prevented us from doing so. In any case, given that this is the first experimental study to explicitly test for the causal effect of regional IO approval on public support for military intervention, our inclusion of an AU treatment condition adds clear value.

Descriptive Findings
We begin the analysis by reporting the descriptive statistics of the dependent variables, pooling across the different conditions of the experimental treatment groups. Figure 1 shows that the mass public generally prefers less costly policies: about 58 percent support assisting France by sending experts (advisers and trainers), 51 percent support providing financial assistance, and only 35 percent support deploying troops. 9 The stronger support for sending experts-the least costly and risky policy-and weaker support for deploying troops-the most costly and risky policy-are consistent with the established wisdom about a "pretty prudent" public that is wary of potentially burdensome and dangerous 7 These benchmarks include a combination of demographic (e.g., age bracket, gender, and education) and non-demographic (e.g., political ideology, party affiliation, and religion) attributes. YouGov generates survey weights to adjust for any remaining lack of representativeness. 8 We left the exact target country unspecified to prevent respondents from being influenced by current news reporting. 9 The data pool respondents from each of the three countries. Disaggregating the data by country produces the same pattern in terms of public support, with deploying troops eliciting the lowest levels of support, followed by providing finance and then sending experts.  foreign military deployments (Jentleson and Britton 1998;Eichenberg 2005). Next, table 1 reports the differences in public support for assisting France in its counterterrorism operation across the United States, Great Britain, and Germany, averaging across the different forms of assistance. Overall, Americans are more pro-intervention than their British and German counterparts. Among Americans, the margin of support for assisting France is about 26 percentage points (i.e., 53.7 percent support, 28.0 percent oppose). The margin of support for Great Britain is 11 points, and for Germany it is 4 points. These trends lend credence to the idea that Americans tend to be more militaristic in their foreign policy attitudes than Europeans (Tomz and Weeks 2013, 860;Everts and Isernia 2015, 98-107).

The Effect of IO Approval on Foreign Public Support
We now turn to the experimental results to evaluate our hypotheses about how IO approval affects foreign public support for contributing to military counterterrorism coalitions. For ease of interpretation, the remainder of the article collapses the support scale to create a binary dependent variable: support or oppose. All of our findings are robust to different specifications of the dependent variable (e.g., using the full 4-point scale and/or including "do not know").
compared to those who read that the IO did not endorse France's intervention. First, we examine the effects of IO approval on public support for our three types of coalition contribution: experts; financial and logistical assistance; and combat troops. As reported in table 2, UNSC approval increases support for these intervention policies by about 5 to 7 percentage points. Similarly, the effect of an AU endorsement is about 5 to 7 points. These effects are statistically significant at the 0.05 level. When comparing the UNSC and AU and aggregating the cross-national data, we find no significant difference in effects. Overall, both IOs have a similar, consistently positive effect on public support for assisting France regardless of the particular type of contribution. Indeed, when the data are pooled across all three types of policies, each IO has about a 6-point effect in raising public support.
Next, we examine the ability of IO approval to raise public support, comparing the three countries in our sample: the United States, Great Britain, and Germany. As reported in table 3, the effect of both the UNSC and AU is generally positive regardless of the country context. However, there are some important differences. UNSC approval appears to be only marginally compelling in the US case: it has a 3.7point effect on American public support, but that effect is not statistically significant. In contrast, the UNSC approval effect has a larger magnitude and is statistically significant for Great Britain and Germany (7.2 and 6 points, respectively). Unlike the UNSC, the AU has a positive and statistically significant effect on public support for contributing to military coalitions across the three countries.
What do these findings tell us about our hypotheses? The aggregate findings offer little evidence for the "null" and "unilateralism" hypotheses (Hypothesis 1 and Hypothesis 2)-that IOs have no effect or even a negative effect on public support. Instead, we uncover ample evidence for the "multilateralism boost" hypothesis (Hypothesis 3)-that IO approval increases public support for contributing to military coalitions, even in the FPR case of counterterrorism. We find some limited support for the policy risk hypothesis (Hypothesis 4): the UNSC effect is strongest for the most costly and risky type of contribution (deploying troops), and weakest for the lost-cost and low-risk option of sending experts; similarly, the AU effect is weakest for the low-risk policy of providing financial assistance. However, the differences in effects are not substantial, and the two IOs had somewhat inconsistently ranked effects across the three policies. Lastly, when pooling the data across the three countries, we find no significant difference between the effects of UNSC and AU approval on public support. This contradicts the hypothesis that the UNSC has a superior legitimation effect compared with regional IOs (Hypothesis 5), and it supports the hypothesis that continent-wide regional IOs with broad and diverse membership can be just as effective for increasing public support (Hypothesis 7).
When examining the data by country, we once again find fairly consistent support for the hypothesis that IO approval increases public willingness to contribute to military counterterrorism coalitions (Hypothesis 3), while the hypotheses that IOs should have a null or even negative effect (Hypothesis 1 and Hypothesis 2) are unsupported. We also find little evidence for the claim that the UNSC is more persuasive in rallying domestic support than the AU (Hypothesis 5), and substantial evidence for the argument that continent-wide regional IOs with broad and diverse membership, such as the AU, can be just as persuasive (Hypothesis 7). In addition, our country-specific data challenge the hypothesis that regional IOs cannot increase public support for contributing to military coalitions among non-member states (Hypothesis 6): AU approval clearly increases public support among all three non-AU members that we included in our analysis-the United States, Great Britain, and Germany. 10 It is also worth highlighting our finding that, for the US public, UNSC approval has a smaller positive effect than AU approval. This might reflect general skepticism about UN performance among American audiences (e.g., Luck 2002), as well as a US tendency to privilege regional over global consent and give particular weight to endorsements from the region targeted by an intervention. Conversely, our finding of a sizeable and statistically significant UNSC approval effect in the cases of the United Kingdom and Germany confirms the conventional wisdom that European publics attach great value to UN legitimation for the use of force (Ku and Jacobson 2003;Krause 2004;Everts and Isernia 2015). What may at first appear somewhat surprising is that UNSC approval did not have the largest effect on the traditionally very pro-UN and pro-multilateralism German public. Instead, the British public saw about an equal, if not slightly greater, movement in response to UNSC approval. This could reflect the long-term impact on British attitudes of Prime Minister Tony Blair's decision to participate in the 2003 Iraq War, which lacked explicit UN approval and resulted in significant domestic blowback among Britons. If one adds to this the British public's war weariness after extended deployments in Afghanistan and Iraq (Richards 2013), the value that a historically "martial nation" (MacDonald 2011) nowadays attaches to UN approval is less surprising.

Party Identification and the Effect of IOs
While the main objective of this article has been to examine the effect of IO approval on overall public support for contributing to military counterterrorism coalitions, in this final section we offer an ancillary analysis of how the IO approval effect intersects with the party affiliation of individual respondents. Research shows that party differences are critical for understanding patterns in public opinion on foreign policy (e.g., Zaller 1992; Berinsky 2009). We have theoretical reasons to expect that these differences could be playing a role in the way that IOs affect public opinion on military intervention as well.
Broadly speaking, right-wing parties based on conservative ideology tend to be more nationalistic and sensitive to sovereignty costs and therefore less likely than their leftist counterparts to embrace multilateralism (Rathbun 2012). In the United States, for example, the Republican administrations under Presidents George W. Bush and Donald Trump sought to severely curtail American involvement with international institutions, both in policy and in rhetoric. Indeed, Busby and Monten (2012) examine polling data for several decades and show that Republican elites have become increasingly opposed to multilateral cooperation in their foreign policy attitudes. Disenchantment with and disengagement from international institutions have surfaced in the right-wing parties of other countries too. In the United Kingdom, for instance, in the 2016 Brexit referendum on continuing membership of the European Union, people identifying as right wing and sympathizing with the Conservative Party voted "leave" in far greater numbers than centrists and leftists (Schumacher 2019). Thus, if right-wing parties and their sympathizers are generally skeptical of international institutions, it is reasonable to infer that individuals affiliated with right-wing parties will be less moved by IO approval for the use of military force.
To evaluate these claims, we analyze whether the IO approval effect differs depending on the respondent's identification with a right-or left-leaning party. Table 4 presents the results, with each country's major right-leaning party displayed first (Republican for the United States, Conservative for the United Kingdom, Christian-Democratic Union [CDU] for Germany); the major left-leaning party second (Democrat, Labour, Social-Democratic [SPD]); and the difference between the two groups last. 11 This analysis reveals that IO approval generally raises public support for coalition contributions regardless of party; however, there are also modest cross-party differences. Across the six potential combinations (UNSC or AU, across United States or United Kingdom or Germany), the effect of IO approval is greater among those identifying with left-leaning parties in four scenarios. For example, UNSC approval increases US support by 8.9 points among Democrats (an effect that is statistically significant), while increasing support by only 3.4 points among Republicans (an effect that is not significant). 12 When the data are aggregated, both the UNSC's 11 For Germany, our "CDU" identification variable also includes supporters of the CDU's Bavarian sister party, the Christian Social Union.
12 This is consistent with the evidence from various studies that Democrats are generally more supportive of multilateral institutions-especially the UN-than Republicans (Everts and Isernia 2015, 120; Fagan and Huang 2019). and the AU's effect on raising public support is slightly greater among those identifying with a left-leaning party, though the difference is only 2 to 3 percentage points. Thus, we find generally consistent, even if not exceedingly strong, evidence for the claim that IO approval has a weaker positive effect among right-wing parties.

Implications
This article has examined the effect of UNSC and AU approval on public support for contributing to military counterterrorism coalitions. Drawing on original survey data from three countries (the United States, Great Britain, and Germany), with an embedded experiment that randomized whether the UNSC and/or AU endorsed the intervention, we found that both IOs have a similar positive effect, noticeably increasing public support for contributing to military coalitions. This finding forces us to reconsider two important theories in the literature on public attitudes toward military intervention. First, our findings challenge the claim that IO approval does not increase already high public support for FPR missions ostensibly aimed at coercing "aggressively threatening" opponents. Our evidence indicates that even in prima facie least likely FPR cases, such as counterterrorism, multilateral validation consistently increases public support for contributing to military coalitions. This finding of a clear positive effect in the counterterrorism case suggests that one might discover an even more noticeable IO approval effect for coalition contributions in pursuit of other objectives where baseline public support is typically lower, such as stabilization in civil wars and civilian protection in mass atrocity situations. Future survey experimental research could usefully test this hypothesis, drawing on the seminal work of Jentleson and Britton (1998) and zeroing in on the contemporary phenomenon of coalition warfare.
Second, we found that UNSC approval does not have a greater positive effect on public support for coalition contributions than endorsements from the AU. This challenges the pervasive argument found in previous scholarship that the UNSC is necessarily better placed than regional IOs to legitimize military interventions and stimulate multinational coalition contributions. Our finding suggests that continent-wide regional IOs with broad and diverse membership, such as the AU, may be just as effective as the UNSC at reassuring foreign publics about an intervention's merits and appropriateness. It may also be the case that popular audiences attach particular weight to the endorsement of IOs from the region targeted by an intervention (e.g., the AU for interventions in Africa), as this might signal a politically valuable form of local consent.
Overall, our findings reveal that, notwithstanding many useful studies to date, there are still numerous aspects of the causal relationship between IO approval and public support for military intervention that cry out for further clarification. Future research could explore in more detail the effect of regional institutional backing on public supportfor example, by investigating possible variation depending on whether broad-based continental organizations (such as the AU and the OAS), or instead subregional organizations with narrower and more homogeneous membership (such as ECOWAS or CARICOM), endorse a military operation. Furthermore, our study focused mainly on public opinion in the aggregate, but future scholarship could explore how individual-level attributes like leader identity and foreign policy disposition mediate the effect of IOs. One might hypothesize, for example, that hawkish leaders would benefit more from IO validation than dovish ones, since the presumption would be that hawkish leaders are war seeking.
Beyond scholarship, our findings have important policy implications for leaders seeking to build military coalitions. IO approval, by our estimates, increases public support for contributing to military coalitions by 5 to 7 percentage points. In all three countries-Germany, Britain, and even the relatively hawkish United States-baseline public support was around or slightly below 50 percent for non-combat contributions and substantially lower for sending combat troops. The bump in public support from IO approval is thus likely to be politically valuable to leaders who are intent on building multinational coalitions (as well as to those who contemplate joining such coalitions), even for FPR missions such as counterterrorism. Our finding that broad-based regional IOs can be as effective as the UNSC in terms of increasing public support likewise has important policy implications. It suggests that leaders craving IO approval to legitimize military interventions and facilitate coalition building have some leeway to "forum shop," seeking the backing of more favorably inclined multilateral bodies-even though endorsements from broad-based regional IOs may not necessarily be easier to obtain than UNSC approval.

Supplementary Information
Supplementary information is available at the International Studies Quarterly data archive.