In 1976, the US Congress halted arms sales to Chile. This paper examines the congressional debate over arms sales to Chile and the political and military consequences of the action. Recent scholarship has largely overlooked the embargo and its implications for regional security dynamics in South America. Initially US sanctions increased Chile's diplomatic isolation and military vulnerability, which made regional conflict more likely. However, Chile's ability to surmount the effects of the embargo eventually increased Augusto Pinochet's independence vis-à-vis Washington. When the Reagan administration began pushing for a transition to democracy, it lacked two key instruments for influencing a military government: weapons sales and security assistance.