Mexico is the main foreign supplier of marijuana and methamphetamines to the United States. It is also a main transit point for cocaine, heroin and other drugs coming to the U.S. from other countries. Mexican cartels have displaced Colombian groups as the dominant cocaine traffickers in the world. Several drug cartels have operations in Mexico, and much of the violence stems from struggles over territory. The cartels' control over specific regions of the country shifts as alliances change and government arrests disrupt the balance of power.