The making of borders in the modern era is often seen as a technical process that reflects the political will of national centers, whereby committees meet to demarcate artificial lines that cut across otherwise ordinary landscapes. More than just a simple process of technical demarcation, however, borders effectively parcel out socio-economic and cultural zones, defining national communities as distinct from those left outside. Fixing the border and introducing institutions to uphold it are therefore crucial to the twin processes of state formation and nation-building. In the case of Turkey’s southern border, this ultimate exercise of territorial sovereignty did not take place until the early 1930s, when border commissions were convened with the French mandatory authorities. The existing literature has largely evaluated this process of border demarcation from diplomatic and political angles, but neglected the impact of ‘illicit’ cross-border transactions on the making of Turkey’s southern border. By drawing upon material from the Turkish, French, and British archives, as well as the local press, this paper addresses this gap by examining how the Great Depression (1929) led to the expansion of illicit circuits globally, and explores the ways in which the introduction of anti-smuggling campaigns came to consolidate the border regimes both in Turkey and French Syria. The global economic downturn in the late 1920s unleashed a war of tariffs across the world, in which the states, urged on by the etatist interventions among their neighbors, began to introduce protectionist measures that were designed to protect local industries and maintain a favorable trade balance. The introduction of heightened tariffs on certain import goods was one such characteristic measure adopted by a number of countries, including Turkey. Because high tariffs created significant price differences from one country to the next, however, they led to the emergence of a highly profitable underground economy across borders, including in the borderland between Turkey and Syria. Here, a sturdy coalition of producers, shop owners, smugglers, trackers, and peddlers, equipped with the necessary fluidity of borderland cultures, began to smuggle into Turkey a range of goods from silk textiles to cigarette papers, to salt, and gold, while funneling out opium and weapons in addition to human-trafficking of ‘undesirable elements’. This paper examines the impact of these borderland mobilities on the making of Turkey’s southern border by exploring the local and bureaucratic responses to a rapidly changing world economic order in the aftermath of the Great Depression.