Abstract
Existing scholarship on alliance burden sharing focuses on explaining why smaller allies often undercontribute relative to their larger partners. However, the literature largely neglects the role played by great-power pressure in shaping burden-sharing outcomes. I argue that rather than being a product of rational free riding, allies' defense efforts are often a response to their patron's threat of abandonment. When a patron can more credibly threaten to reduce its protection, and when doing so would impose serious costs on allies, the former is better positioned to elicit burden sharing. I test the theory using data on allied burden sharing in US alliances from 1950 to 2010. The results show that allies exhibit higher levels of burden-sharing efforts when they are geographically vulnerable to attack, whereas allies exhibit lower levels of burden sharing when they are in geostrategic locations valuable to the United States.
