Before addressing its central concern—the convergence of science, war, institutions, and politics in the postwar period in France and the United States—, this essay evokes how scientific knowledge had been of importance to warfare and economic elites in the preceding centuries. In the 1940s and 1950s, scientific activities were profoundly redefined. A culture of laboratory solutions, of calculus, and management won the day. For the scientists, that meant versatility and a willingness to work between disciplines and métiers and to confront the nation's main concerns. It also led to increasingly technocratic versions of politics. Due to science, the state became a managerial apparatus, a "modernizer" arbitrating among different scenarios. Contrary to what happened in the United States, science was not center stage in France in the 1940s and early 1950s. The habitus of scientists was that of the prewar period, and they were still not technique-oriented. They had a more cultural definition of their trade and were not opportunists whose aim was to become pragmatically efficient in the world of business and military action. From the mid-1950s, things started to evolve due to a strong economic recovery and because French scientists had now caught up with the latest developments. The final break, however, occurred in France only when de Gaulle abandoned the Algerian war and elected for an autonomous nuclear deterrence system. By putting la stratégie de l'arsenal at the core of national development, de Gaulle significantly transformed French science, society, industry, and the military.