Why do states facing high levels of international threat sometimes have militaries that are heavily involved in politics and at other times relatively apolitical, professional militaries? I argue that the answer to this puzzle lies in a state's history of 'acute' international crises rather than its 'chronic' threat environment. Major international crises lead to professionalization and de-politicization of militaries in both the short- and long-term. International crises underscore the need for the military to defend the state and highlight military deficiencies in this regard. Accordingly, major international crises lead to military professionalization and withdrawal from politics in order to increase military effectiveness. This effect persists years, and decades, later due to generational shifts in the officer corps. As the "Crisis Generation" of officers become generals, they bring with them a preference for professionalization and de-politicization. They guide the military towards abstention from politics. I test this theory using a new global dataset on military officers in national governing bodies from 1964-2008 and find strong support for the theory. Major international crises lead to two waves of military withdrawal from government, years apart. Further statistical analysis finds that this effect is most strongly felt in the non-security areas of governing, while in some cases, international crises may lead to militaries increasing their involvement in security policy-making. Further, international crises that end poorly for a state — i.e., defeats or stalemates — are found to drive more rapid waves of military withdrawal from government. The statistical analysis is supported by a case illustration of civil-military relations in the People's Republic of China, which demonstrates that the crisis of the Korean War (1950-53) led to two waves of military professionalization and de-politicization, decades apart. The first occurred immediately after the war. The second wave, occurring in the 1980s, involved wholesale military withdrawal from governing bodies, which was made possible by the ascent of the "Crisis Generation" of officers in the military, who had served as junior officers in the Korean War, decades prior.