This article reviews twenty years of public interventions in Chad’s water sector with the support of a development cooperation agency. The program’s main contribution arises from a radical shift in donor and government approaches, from a vision of water supply as an end in itself to a systemic vision of water as a means of securing pastoral mobility. This shift translates into a commitment to large-scale, long-term projects, better understanding of their social and economic context, supporting customary institutions, and to the promotion of complementary production systems. This experience offers useful lessons on increasing security in the Sahel, and on meeting the challenge of modernizing pastoral systems—by reinforcing rather than dismantling them.