As noted in the Introduction, it is far from clear why subnational jurisdictions like states and provinces would choose to fight over water, especially for prolonged periods of time. Enmeshed in deep political, social, and economic relationships, these jurisdictions and the politicians who lead them would seem to have much more to gain from cooperation than conflict. In this chapter, I explain the emer¬gence and persistence of subnational conflict and cooperation over water in terms of two oppositional factors, sectional identity and political opportunity structure. As I describe further in this chapter, persistent interjurisdictional water conflict usually results from the conflation of shared water resources and sectional identity. In particular, subnational political leaders often seek to gain political support from geographically concentrated ethnic, linguistic, or other groups by alleging that neighboring groups are effectively stealing water at their constituents’ expense. Because of the unique economic importance and emotional resonance of water, such allegations are highly effective in mobilizing political support, and successive subnational politicians can find it expedient to engage in confrontational rather than cooperative behavior. Using water as a tool for political mobilization can also help subnational politicians achieve other objectives, including enhancing their autonomy, distracting from other political challenges, and outflanking opposition parties. On the other hand, where the political opportunity structure in a given country enables third parties to form alliances with national political elites, these alliances create a constituency for strong interjurisdictional institutions that can prevent and resolve conflicts over shared water resources. This chapter proceeds in three sections. First, it explains how the conflation of sectional identity and shared water resources sustains interjurisdictional conflict over long periods of time. The desire of subnational politicians and geographically concentrated water user groups to seek broader support for their objectives leads them to link water-sharing issues to sectional identity. The second section describes how, provided they enjoy access to the political process, third parties can help to induce collective action in shared river basins.