Coalition Strength Depends on Effort, Not Just Military Size

What Question Do the Authors Ask?

Brenton Kenkel and Kristopher W. Ramsay ask what determines the effective power of military coalitions. They challenge the intuitive idea that a coalition’s combat strength is simply the sum of its members’ capabilities, arguing instead that incentives—free riding and competition among partners—shape how much effort states actually contribute to coalition fighting and bargaining.

A Unified Model of Crisis Bargaining and War-Fighting

The authors build a structural model that links crisis bargaining (how states negotiate during disputes) to battlefield performance when disputes escalate to fighting. The model embeds incentives for partners to exert effort or shirk, and it maps observable dispute outcomes back onto latent parameters that capture each country’s "force multipliers" (how effectively material resources translate into military power) and "audience costs" (domestic political costs of backing down).

Structural Estimation Using Dispute Outcomes

Kenkel and Ramsay use data on the escalation and outcomes of international disputes to structurally estimate the model’s parameters. By fitting the model to observed patterns of who escalates, who wins, and how coalitions behave, they identify how demographic, economic, political, and strategic factors shape states’ force multipliers and incentives to contribute effort.

Key Findings

• Coalition power is not additive: free riding and intra-coalition competition mean partners’ contributions can fall short of their raw capabilities.
• Demographic and economic characteristics (for example, population and economic capacity) are the most important predictors of military effectiveness as captured by force multipliers.
• Regime type and geopolitical considerations influence effectiveness as well, but play smaller roles than demographics and economics in the authors’ estimates.
• The estimated audience cost parameters help explain variation in bargaining behavior and willingness to escalate.

Counterfactual Simulations and Historical Tests

The fitted structural model is used to simulate counterfactuals, letting the authors test historical claims about coalition dynamics. For example, the simulations indicate that earlier American involvement in major twentieth-century conflicts could plausibly have altered outcomes—even after accounting for strategic free riding by allies—because of the United States’ high force multipliers in the model.

Why This Matters

This work connects theories of international bargaining with concrete measures of battlefield effectiveness and provides a tool for assessing how coalition composition and incentives shape crisis outcomes. Policymakers and scholars can use these structural estimates to evaluate alliance management, burden-sharing, and the likely impact of changes in members’ capabilities or incentives on future conflicts.