Violent crimes committed by drug trafficking organizations in Mexico often involve firearms, and a 2009 Government Accountability Office (GAO) report found that many of these firearms originated in the United States. The Department of Justices Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) and the Department of Homeland Securitys Immigration and Customs Enforcement (ICE) have sought to stem firearms trafficking from the United States to Mexico. This book examines, among other things, the origin of firearms seized in Mexico that have been traced by ATF; the extent to which collaboration among U.S. agencies combating firearms trafficking has improved; and the extent to which the National Southwest Border Counternarcotics Strategy measures progress by U.S. agencies to stem firearms trafficking to Mexico. Furthermore, the book examines the activities undertaken by U.S. agencies to build partner capacity to combat firearms trafficking and the extent to which they considered key factors in selecting the activities; and progress the United States has made in building such capacity.