I present a bargaining theory of strategic cooperation--which I define as voluntary, deep, and enduring cooperation--that focuses on the influence of credible commitment problems to explain variation in the qualities of strategic cooperation. I argue that variations in a relationship's qualities of cooperation are primarily explained by credible commitment problems at the international and domestic levels. I identify three sources of credible commitment problems at the international level (spoiler problems, competitor problems, and other international conditions) and five sources of credible commitment problems at the domestic level (disinterest, trust, reconciliation, state capacity, and political unification problems) that might undermine a relationship's qualities of cooperation. I test my theory against a restricted universe of cases that is comprised of the U.S. wars from World War II to present, arguing that the two most likely sources of domestic credible commitment problems in these U.S. postwar contexts are reconciliation problems and political unification problems within the former U.S. adversary. My case analyses largely support my theory. This has substantial policy implications. My theory should now be tested on a broader array of strategic cooperation contexts to improve its generalizability.