The Strong, The Weak, and The Honored: Examining the Decline in Honored Alliances Post-1945

The rate at which states defend their allies in war has dropped from 81% during 1816-1944 to 7% in 1945-2016. I attribute the decline in honored alliances to a dramatic shift in the military capability of alliances. Contrary to the popular belief that alliances have become stronger after 1945, I find that the post-1945 international system also witnessed a significant increase in the number of weaker alliances. This bimodal distribution of alliances produced a pattern where alliances were either violated upon being attacked or never attacked in the first place, leading to a decline in the rate at which alliances were honored. I support my argument using alliance data over two centuries. This research advances our understanding of military alliances by documenting a polarization of alliances in terms of their military capability post-1945 and by providing an explanation for an empirical puzzle—a sharp decline in the rate of honored alliances after 1945.