My dissertation focuses on the establishment of Republican Turkey and the League of Nations Mandates in present-day Syria, Lebanon, and Iraq following the fall of the Ottoman Empire. I seek to develop a comparative inquiry into how elite and non-elite sectors of late Ottoman society coped with transitions to these post-Ottoman regimes in the decade between 1918 and 1928. I explore three particular processes that underpinned this transitional period: Population movements between territories which eventually became 'homelands' for different post-Ottoman nation-states, cross-border disputes over government pensions and ownership of land and property, and the creation of new (official and informal) credentials for state employment. I argue that cooperation between the Ankara government and the French and British Mandate governments represents a crucial and overlooked component in the construction of post-Ottoman Turkey, Syria, Lebanon, and Iraq. The cooperation between these post-Ottoman regimes facilitated the division of late Ottoman society into separate national units through an intense exchange of empirical knowledge. This cooperation invites a transnational and transregional perspective on the nation-making processes that shaped Turkey, Syria, Lebanon, and Iraq in the interwar period. My project highlights that Turkey and other former Ottoman territories did not follow completely separate historical trajectories once the Ottoman Empire collapsed, and thus reintegrates Turkey into the interwar history of the Middle East. By exploring the relations between the Ankara government in Turkey, the French mandate regime in Syria and Lebanon, and the British mandate regime in Iraq, my dissertation also places the French and British Mandates on highly useful grounds of comparison that can yield fresh insights into the multiple meanings of empire and the mechanisms on which it relied to function over different landscapes in time.