States facing internal conflict often receive military assistance from the United States, but the relationship between such assistance and state capacity is not well known. In this working paper, post-doctoral fellow Oeindrila Dube and co-author Suresh Naidu offer new research on the links between military assistance and political violence. They find that increased U.S. military aid to Colombia, a haven for narcotics trafficking long-plagued by guerrilla warfare, increases paramilitary violence but has no effect on guerilla violence. With significant implications for U.S. policy in weak and conflict-ridden states, the evidence indicates that the effectiveness of military aid, which is intended to bolster the weak state against violent groups, is undercut by collusion between the military and illegal armed groups. Further, the authors expose that this collusion means that foreign assistance directly enables illegal groups to perpetuate political violence and undermine democratic institutions, such as electoral participation.