Having dealt with the implantation of administrative structures, I turn now to the borderlands as sites of regulation. My aim is, firstly, to reveal in much greater detail the different ways in which the border dynamics became woven into processes of state-making during the interwar period. In the Senegambia, the fundamental issue was the management of mobility, which ultimately proved conducive to the development of relatively open borders. In the trans-Volta, the energies of state officials were concentrated on regulating commodity flows. These contrasting imperatives involved a different range of actors, including traditional authorities whose own relationship to the central state was forged through the daily routines of border control. Secondly, the chapter seeks to account for some of the differences that unfolded on the two sides of the borders in question, resulting not just from divergent government priorities, but also from the fundamental incompatibilities between them. Although there were moments of selfless co-operation – mostly witnessed at times of crisis such as armed rebellion or war – colonial administrations tended to regard each other as competitors. While officials suspected their counterparts of conspiring with border populations to frustrate their own efforts, they also expressed sympathy with populations that were subject to the neighbouring administration.