Measured Strength: Estimating the Strength of Alliances in the International System, 1816-2000

Alliances play a critical role in the international system and understanding the determinants and consequences of their strength is an important task. Even though many have argued that the strength of an alliance is theoretically determined by both the power of the signatories involved and the formal terms of the agreement, using these insights to measure the strength of alliances is difficult for many reasons. Using a statistical measurement model, we estimate the strength of all alliances signed between 1816-2000 along two theoretically derived dimensions: the strength of the signatories involved, and the strength of the formal terms of the alliance. In addition to estimating the strength of every alliance in these two dimensions, our Bayesian latent variable model also allows us to: characterize the relationship between the two dimensions of alliance strength, identify how observable characteristics relate to each of the recovered dimensions, and, perhaps most importantly given the difficulty of the task, document how precisely we are able to use the measures to estimate alliance strength. By generating estimates of every alliance signed between 1816 and 2000, we not only provide scholars with an empirical characterization of alliance strength in two dimensions of conceptual interest that should prove vital for future studies of international alliances, but the flexibility of our measurement model also offers additional opportunities to refine and extend our measure.