Relations between the United States and Mexico have rarely been easy. Ever since the United States invaded its southern neighbor and seized half of its national territory in the 19th century, the two countries have struggled to establish a relationship based on mutual trust and respect. Over the two centuries since Mexico’s independence, the governments and citizens of both countries have played central roles in shaping each other’s political, economic, social, and cultural development. Although this process has involved—even required—a great deal of cooperation, relations between the United States and Mexico have more often been characterized by antagonism, exploitation, and unilateralism. This long history of tensions has contributed to the three greatest challenges that these countries face together today: economic development, immigration, and drug-related violence.