WASHINGTON (AP) -- Confronting a security threat on America's doorstep, President Barack Obama launched a swift diplomatic mission to Mexico Thursday to show solidarity with a troubled neighbor -- and to prove that the U.S. is serious about halting the deadly flow of drugs and weapons. The escalating drug war in Mexico is spilling into the United States and onto Obama's lap as a foreign crisis much closer than North Korea or Afghanistan. Mexico is the main hub for cocaine and other drugs entering the U.S.; the United States is the primary source of guns used in Mexico's drug-related killings. Mexican President Felipe Calderon's aggressive stand against drug cartels has won him the aid of the U.S. and the prominent political backing of Obama -- never as evident as on Thursday, when the popular U.S. president is sure to stand with Calderon on his own turf and note his courage. (22 images)