The U.S. Supreme Court ruled Tuesday that the parents of a 15-year-old boy cannot sue the federal agent who shot him by firing across the border separating the United States and Mexico — a case that inflamed tensions over border security. The ruling was a defeat for the parents of Sergio Hernandez Guereca, who was on the Mexico side when he was killed in 2010 by a Border Patrol agent who fired from the U.S. side of the boundary separating El Paso, Texas, from Juarez, Mexico. The teen, a Mexican national, was playing with three friends in the cement culvert...