Between the Ottomans and the Entente is the first social history of the First World War written from the perspective of the Arab diasporas in the United States, Brazil, and Argentina. The war between the Ottoman Empire and the Entente Powers placed the half million Syrian, Lebanese, and Palestinian migrants living abroad in a complicated geopolitical predicament. As Ottoman citizens living in a pro-Entente hemisphere, Arab migrants faced new demands for loyalty by their host societies; simultaneously, they confronted a multiplying legal regime of migration restriction, passport control, and nationality disputes designed to claim Syrian migrants while also controlling their movements. This work tracks the politics and activism of Syrian migrants from the 1908 Young Turk Revolution through the early French Mandate period in the 1920s. It argues that Syrian migrant activists opposed Ottoman rule from the diaspora, collaborating with the Entente powers because they believed this war work would bolster the cause of Syria’s liberation from Unionist rule. Instead, the Entente Powers used support from Syrian migrant communities to bolster colonial claims on a post-Ottoman Levant. This work captures a series of state projects to claim Syrian migrants for the purposes of nation-building in the Arab Middle East, and the efforts of Syrian migrants to resist the categorical schema of the homogenous nation-state and policies of partition and displacement.