The deteriorating European security environment underscores the continuing relevance of NATO’s collective defence commitments. Since NATO is a military alliance comprising 32 electoral democracies, the commitment to defend any member relies on the domestic politics of its members. For these commitments to be credible, the alliance requires domestic political consensus and public support for defending allies that become targets of external aggression. However, NATO’s annual surveys reveal striking cross-national heterogeneity among allied states in public willingness to honour their collective defence commitments under Article 5. Despite the increased interest in examining the microfoundations of alliance politics, we still lack systematic explanations for why the publics in some member states are more hesitant to defend allies than in others. To address this important gap, we will analyse data from a unique multi-year survey conducted on nationally representative samples across all NATO countries. Using multilevel statistical modelling, our study will provide the first evidence on country-and individual-level factors that explain the large cross-national variation in public views on collective defence. Our findings will contribute to scholarly debates on the role of public opinion in military alliances and the pressing policy discussions about NATO’s cohesion and credibility.

Despite a broad consensus in literature and policy circles that NATO member states free-ride, credible causal evidence remains very scarce. A straightforward assessment of the free-riding hypothesis is whether countries reduce military spending after joining NATO relative to otherwise comparable non-members. We address this question by, first, selecting ex ante post-1991 entrants and geopolitically comparable non-members and, second, estimating membership effects using modern panel difference-in-differences methods that accommodate staggered adoption and heterogeneous treatment effects. Furthermore, to probe anticipatory adjustment, we also analyze the NATO Membership Action Plan (MAP) as an earlier potential treatment. Across specifications, we find no evidence that NATO membership induces systematic post-accession cutbacks and suggest a lack of evidence of anticipation. Going beyond null-hypothesis testing, we derive one-sided bounds that exclude large medium- and longer-term average spending reductions. These results challenge the prevailing wisdom as post-1991 NATO accessions do not appear to have generated meaningful moral hazard-driven cutbacks, and any such incentives were likely constrained or offset by domestic politics and alliance pressures to maintain or increase spending.

The literature shows that publics are generally reluctant to raise defence spending in peacetime, yet attitudes in frontline states remain underexplored. Evidence is also scarce on how allied force-posture signals shape mass opinion. We study Lithuania under heightened threat after Russia’s 2022 invasion of Ukraine and Germany’s decision to station a brigade (in Lithuania). Moving beyond Western European samples and U.S.-centric scenarios, we field a large, preregistered online survey experiment (N=3,200; April 2026) that varies credible, country-specific posture cues: a U.S. drawdown, a German increase, their combination, and a status-quo control. The outcomes are support for national defense spending, willingness to defend, and policy trade-offs. Embedding theory-driven mediators (readiness, fairness, loss salience, and entrapment risk) allows us to distinguish free riding from reciprocity and to assess asymmetries in how losses and gains are processed. The study contributes micro-level evidence on burden-sharing preferences in an exposed ally and improves external validity through realistic vignettes tied to actual deployments. We will share preliminary findings at the September 2026 conference.

Russia’s ongoing invasion of Ukraine has underscored the importance of collective defence, long anchored in the North Atlantic Treaty Organisation (NATO) and its mutual defence clause, Article 5 of the North Atlantic Treaty (NAT).However, a similar mutual assistance/defence clause also exists within the European Union (EU). Article 42(7) of the Treaty on European Union (TEU) commits the twenty-three EU Member States that are also NATO Allies to assist one another in the event of armed aggression. These provisions have received relatively little attention from legal scholars. In a context marked by heightened strategic concerns about conventional attacks on NATO and/or EU territory, as well as uncertainty over the United States’ commitment to NATO and European security, this article examines how Article 5 NAT and Article 42(7) TEU differ in terms of their legal obligations, scope, and credibility as mechanisms of collective defence. Adopting a law-in-context approach, this article analyses and compares the overlapping collective defence clauses of NATO and the EU. It clarifies their legal scope and situates them within the broader geopolitical and strategic context in which they operate, drawing on international relations and security studies. Two case studies are examined: 1) the invocation of Article 5 NAT following the 9/11 terrorist attacks, and 2) the activation of Article 42(7) TEU in response to the November 2015 Paris attacks. The analysis provides important insights into the enduring legal and institutional challenges of advancing defence cooperation in Europe.