Shimizu Ikutarō (1907–1988) is one of the most controversial postwar Japanese intellectuals. His transition from the icon of the Anpo protests to an advocate of a nuclear Japan has been considered an intellectual conversion (tenkō). Instead of revisiting the notion of conversion, this study shows that his wartime thoughts—bottom-up nationalism in particular—continued to influence Shimizu’s postwar writings and activism on both conservative and liberal sides. Shimizu delineated his historical concept of how ordinary people in Meiji and Taisho Japan had contributed to the development of a modern society and called for the construction of a new system. Endorsing Japan’s wartime efforts, Shimizu strove to center nationalist energies by ordinary Japanese on his concept of a new Japan. However, Shimizu’s adherence to bottom-up movements in wartime and postwar Japan reflects his problematic interpretation of Japanese history. Neglecting Japan’s nationalistic path to colonial violence, his writings on the society and culture of wartime and postwar Japan affirm grass-root nationalism as Japan’s key to modern development. This line of thinking was later associated with anti-American nationalist movements in the 1950s. His notion of civil society movements soon encountered a highly nationalistic project of a nuclear Japan in the 1970s.