S.M. Kartosuwiryo, famed leader of the long and bloody Darul Islam rebellion which began in West Java in 1948, was a strong supporter of the Indonesian independence struggle and a champion of the Indonesian Republic proclaimed in 1945. This article seeks to understand how it was that Kartosuwiryo came to oppose that very Republic with such violence in 1948–49. Many scholars have sought to explain the origins of the Darul Islam movement in terms of Kartosuwiryo's fanatic Islamist ambition. However, a detailed examination of the circumstances of the revolt's gestation and outbreak indicates that it was a consequence of a complex interplay of historically contingent circumstances rather than any ideological fixity.