Relations between Cuba and Mexico may be crumbling. Cuban leader Fidel Castro says Mexico — Cuba's most important Latin American ally — undermined him last week at a U.N. conference in Monterrey. Castro says he was pressured to leave before President Bush arrived. Both Mexico and the U.S. deny the charge, but Castro says he has proof. NPR's Gerry Hadden reports from Mexico City on the strained relations between Mexico and Cuba.