<Ul> <Li> <P> Border Patrol patrolling the Rio Grande in an airboat in Laredo, Texas </P> </Li> <Li> <P> The start of the border fence in the state of New Mexico--just west of El Paso, Texas </P> </Li> <Li> <P> U.S. Border Patrol helicopter along El Camino del Diablo, Arizona--Sonora border, 2004 </P> </Li> <Li> <P> Border between Nogales, Arizona, on the left, and Nogales, Sonora, on the right </P> </Li> <Li> <P> On the left: Nogales, Arizona; on the right, Nogales, Sonora </P> </Li> <Li> <P> Beach in Tijuana at the border </P> </Li> </Ul> <Li> <P> Border Patrol patrolling the Rio Grande in an airboat in Laredo, Texas </P> </Li> <P> Border Patrol patrolling the Rio Grande in an airboat in Laredo, Texas </P> <Li> <P> The start of the border fence in the state of New Mexico--just west of El Paso, Texas </P> </Li>

Which two countries border mexico on its southern border