This paper examines the development of the Iraq-Nejd and Iraq-Syria boundaries during the British Mandate period from the perspective of indigenous populations and state officials who lived and worked in the borderlands. Whereas other scholars of the Iraqi state-building period explain Iraq’s borders as functions of French and British imperial deal-making and arbitrary “lines in the sand,” I approach them as institutions that emerge from inimitable processes of reconciling abstract political or strategic objectives with the real-life conditions of government at the very limits of state authority. By focusing on the underlying materialities of rule described in British intelligence files and maps, League of Nations reports, and geographies or first-person accounts by Arab statesmen involved in Iraq’s early history, I demonstrate how the boundary formation process depended on policing the movement of transnational Arab Bedouin tribes in a border landscape of wells, grazing land, and caravan routes in the Syrian and northern Arabian deserts. My first case study involves the 1925 forced resettlement of hundreds of Sunni Ikhwan “refugees” from Nejd into northern Iraq, because their disruptive presence along the frontier exacerbated tensions between Iraq and Ibn Saud, and appeared to lower British prestige among Iraqi tribes. The second case study is a series of informal border conferences between provincial Iraqi, Syrian, and British officials in the late 1920s, the purpose of which was to mitigate security problems and unregulated border crossings arising from the discrepancy between the “de facto” boundary that Britain enforced well to the west of the official 1920 “Leachman Line” between Iraq and Syria. I show Iraq’s boundaries to be a process, not a fact, of the Mandate era, one involving low or mid-ranking British military and intelligence officers, Arab provincial governors and police, and Bedouin tribal shaykhs of the northern Arabian Desert and Syrian Desert. Outstanding political issues such as the rise and expansion of Ibn Saud’s kingdom in Nejd, or British territorial claims to eastern Syria, were addressed obliquely through the policing of tribal movement, raiding, and territoriality. These findings necessitate the reassessment of our field’s understanding of refugees and the politics of resettlement in the inter-war Middle East, the effect of Wahhabi, Saudi, and Hashemite territoriality and ideology on the development of Iraq’s current boundary with Saudi Arabia, and the implications of using local, indigenous proxies to carry out centuries-long rivalries between European empires in the seminal period of Middle East state formation.