Modeling alliance membership in a three‐dimensional space

Both Altfeld (1984) and Morrow (1991) offer theoretical foundations that explain why alliances form. Security concerns and autonomy desires are the driving force for the pursuit of allies. While this response explains why states become allies, it does not address which do. To explain with which potential allies states choose to share alliance membership, I construct a three‐dimensional space in which security, autonomy, and political system structure are each represented as one of the three dimensions of ally choice. Every state that shared alliance membership with any other given state from 1946--1992, on a yearly basis, is assigned a unique, tri‐component point in the three‐dimensional space. The distances in the space between each state and all other states’ tri‐component points are calculated, and this distance is then incorporated into a probit estimation of ally choice. By examining the role that security and autonomy trade‐offs and level of political system structure similarity play in determining ally choice, I conclude that two states are more likely to share alliance membership the more symmetrical they are in security and autonomy considerations and the more similar they are in political system structure.