Facundo Cabral, an Argentine singer-songwriter who was one of the most eloquent voices of protest against military dictatorships in Latin America from the 1970s onward, died on Saturday, shot to death while on tour in Guatemala. He was 74 and lived in Buenos Aires.Mr. Cabral was killed when the car in which he was a passenger, on its way to the airport in Guatemala City, was ambushed by unidentified gunmen in three vehicles. His road manager, Davíd Llanos, and a concert promoter and nightclub owner from Nicaragua, Henry Fariña Fonseca, were seriously wounded in the attack.The death of Mr. Cabral, who in 1996 was designated a “worldwide messenger of peace” by the United Nations, caused consternation throughout the Spanish-speaking world. President Hugo Chávez of Venezuela sent a message via...