Rosella Cappella Zielinski, Associate Professor of Political Science, Director of Graduate Studies, and Associate Professor of the Pardee School of Global Studies at Boston University, has co-authored a new article, “When Do Militaries Fight Battles Together?”, published in Security Studies (2026) with Ryan Grauer of the University of Pittsburgh. The article takes on a surprisingly understudied question in international security: when wartime allies decide to actually fight side-by-side on the battlefield, as opposed to merely fighting on the same side of a war. While the literature on wartime coalitions is extensive, the specific decision to deploy forces together in the same operational battlespace — forming what the authors call a “battlefield coalition” — has received far less scholarly attention, despite its significant consequences for how wars are fought and won.

The article’s central argument is that two factors, operating close to the front line of combat, determine whether battlefield coalitions form: shared stakes and logistical capacity. Shared stakes refers to whether all potential coalition partners agree that a looming operation is high-value and that no single belligerent could succeed alone. Logistical capacity refers to the collective ability to move troops and materiel to the battlefield and sustain them for the duration of the fight. When both conditions are present, battlefield coalitions are especially likely to form. When both are absent, they almost certainly will not. The article’s novel contribution is demonstrating that logistical capacity can, under certain conditions, overcome disagreements about stakes — making it a particularly powerful driver of coalition formation.

The stakes are significant. Drawing on an original dataset covering 371 major land battles in multilateral interstate wars between 1900 and 2003, the authors show that 228 battlefield coalitions formed and fought together during this period. Their research showed that battlefield coalitions win 53% of their battles compared to 46% for solo belligerents, and they suffer roughly 4% fewer casualties per battle — a difference equivalent, in a hypothetical engagement of 40,000 troops, to more than 1,500 fewer dead and wounded. Understanding why these coalitions form when they do is therefore not merely a theoretical exercise; it has direct implications for how military planners and policymakers approach alliance management and combined operations.

To test their theory, Zielinski and Grauer turn to the World War II Burma Campaign, examining two Allied attempts at combined operations involving British, American, and Chinese forces. The first, Operation Anakim, was a planned multipronged assault on Japanese positions that the Allies abandoned in 1943. The second, the strike at Myitkyina, was executed successfully in 1944. The contrast between the two is analytically powerful: the belligerents, their regime types, their relative power, and their familiarity with one another remained essentially constant across both cases. What changed was the Allies’ logistical capacity. By late 1943, improved rail infrastructure, expanded airfields, and better air supply operations gave the Allies the means to move and sustain forces in ways that had been impossible during the planning for Anakim. That improved capacity made a battlefield coalition viable even as British disagreement about the strategic value of operations in Burma persisted.

The article’s findings carry important implications for contemporary security policy. The authors note that since 1945, the United States and its partners have operated in largely uncontested logistical environments, which has led planners to underweight logistical considerations in thinking about combined operations. A major conflict in the Indo-Pacific would look very different — featuring contested sea and air lines, terrain hostile to easy supply operations, and an adversary specifically designed to degrade U.S. power projection. In that context, the Burma Campaign offers a cautionary and instructive parallel: the last time the United States fought alongside partners in an environment where logistics were genuinely contested. The authors argue that investing in collective logistical capacity — not merely each partner’s individual capability — will be essential to making future battlefield coalitions viable when they are needed most.

The full chapter can be read here.

To learn more about Professor Rosella Cappella Zielinski’s work and achievements, visit her faculty profile.