From Syria and Iraq to Afghanistan, Ukraine, India, and Somalia, recent episodes of civil conflict highlight three prominent phenomena: (1) the United States is increasingly funding border control initiatives abroad, with an eye toward countering transnational, non-state threats; (2) borders are hardening in response, with counterinsurgents’ efforts to secure borders against transnational mil- itancy partially driving this trend; and (3) because rebels often exploit transnational safe havens, border control efforts that interdict transnational militant operations have important implications for the microdynamics of civil conflict. In this dissertation, I provide empirical support for these claims using a multi-method design. First, I show that aid and training programs for border se- curity, which the United States has created since 2001, have induced developing recipient states to harden their borders. Second, I examine how border fortification during Operation Iraqi Free- dom shaped rebel violence during the conflict. I posit a fortification dilemma, whereby border fortification reduced insurgent capabilities but spurred insurgents to build closer ties with civilian communities in the Iraqi borderlands. Third, I examine the social and economic consequences of border fortification in Afghanistan between 2008 and 2014. I identify a trade-off between security and corruption incumbent in border control strategies. By expanding state capacity in peripheral border regions, fortification improves government security provision and the perceived legitimacy of the central state. However, by dislocating traditional, cross-border markets, fortification spurs economic resistance and corruption by informal economic elites. Together, these papers underscore causes and consequences of border hardening in fragile, conflict-affected settings.