The world's two great powers, China and the United States, are competing over international order—that is, the rules, norms, and institutions that regulate relations among states and societies. China's ruling party is discontented with several features of the liberal international order that the United States and its allies constructed after World War II. Sino-American struggles over the content of institutions suggest that great powers compete to control international institutions. Such competition is especially intense when great powers have different domestic regimes, such as democracy and autocracy. Power and security are at stake because different types of international orders can grant material and social advantages to different types of states. My ecological theory of competition over international institutions makes three claims: (1) international institutions select for one regime type over alternatives; (2) the typical government believes that it is in its core interest to preserve its regime type; and (3) great powers can shape international institutions to select for their own regime type. I demonstrate these dynamics in great power relations in the interwar period (1919–1939).