I examine a relatively underexplored aspect of Japan's early postwar history and seek to explain why attempts to establish a Japanese-style central intelligence agency (JCIA) in the 1950s were unsuccessful. I evaluate three competing explanations drawn from the level of international politics, focusing on US power resources and influence as well as liberal and constructivist styles of analysis—alliance politics, sectionalism, and the norm of antimilitarism—in order to shed light on the historical origins of Japan's intelligence apparatus, which is relatively underdeveloped and underfunded compared to other middle powers. It highlights the primacy of domestic factors over structural causes in explaining the decision not to establish a JCIA. In particular, I argue that the JCIA proposal failed primarily because of attacks on important proponents that, while sometimes driven by seemingly rational organizational interests, were nevertheless legitimated by growing antimilitaristic sentiments shared by elites from the political center to the left of the ideological spectrum. The newly emerging norm of antimilitarism was predicated largely on a fear of constraints on recently acquired civil and political liberties. These fears, manifested most prominently in vocal Diet and media opposition, were compounded by the norm of secrecy—an important element of intelligence activities—which served to heighten further speculation about the malign intent of postwar Japan's reconstituted intelligence system.