Alliance Burden-Sharing in System Polarities

As we move into the third decade after the end of the Cold War, scholars and commentators continue to ask how newly defined roles of various U.S. allies will correspond to their levels of burden sharing. In order to answer this question, we draw on neorealist and economic models to develop theoretical expectations about the relationship between system polarity and alliance burden sharing. Using the Alliance Treaty and Obligations (ATOP) data to measure alliances, and military expenditures and GDP data to fashion a burden sharing index, we examine the effects of major power and minor power alliances with a system leader under multipolarity, bipolarity, and unipolarity from 1885 to 2001. Consistent with the expectations, allies' burden sharing demonstrates no clear pattern under multipolarity; decreases for U.S. allies and major powers allied with the USSR, but increases for minor powers allied with the USSR during bipolarity; and decreases for minor powers allied with the U.S. while having no effect on major powers allied with the U.S. under the current unipolar system.