MIAMI — With his back to the beach where he had landed in an invasion to liberate his country, 16-year-old Humberto Martinez was just minutes away from execution. “Hold up, El Caballo wants them captured,” Martinez recalled a lieutenant saying, relaying orders from Fidel Castro — nicknamed “the Horse" — to the improvised firing squad that was getting ready to execute seven members of the Brigade 2506 captured during the Bay of Pigs invasion. He had no time to think of anything before death. He was too young to fear it, said the now 76-year-old Martínez, who after six decades still has fresh me...