Ibn Saud of Najd, British Mandate officials, and the Iraqi government established the terms of the Iraq-Najd boundary in 1922, but the competition for territory in the Arabian Desert started as early as World War I and continued as late as 1932. This paper studies police and military posts along the Iraq-Najd frontier to better understand how British and Arab forces sought to establish or determine de facto and de jure authority over tribal populations before and after the ratification of formal boundary agreements. I use India Office memos, Royal Air Force intelligence files, and Foreign Office maps and correspondences between Ibn Saud and the British and Iraqi governments. These documents show how British and Saudi military outposts and patrol routes were used to control tribal mobility and force the settlement of tribal populations by insinuating themselves into the infrastructure of every-day tribal life, such as caravan routes, oases and wells, or trade depots and caravanserai. In my analysis, I describe how with the passage of time, these military outposts unintentionally became border or police outposts with a different status and role in Iraqi governance and regional politics. These outposts, often founded on short-term security goals and uncoordinated military actions against stateless tribes, became trading pieces during negotiations over the location of the Iraq-Najd boundary. One consequence of this negotiation process was the nationalization of specific tribes as either “Iraqi” or “Najdi” based on their proximity to these military-cum-border posts. In this way, these posts play an integral role in building the Iraqi state, and populating it, during the Mandate era.