On a chilly day in December 1981, several thousand soldiers descended on the village of El Mozote, in El Salvador. By the following evening, more than eight hundred civilians were dead. The troops massacred an entire village, allegedly because they were anti-government guerrillas.

Among the many horrible things about the myriad atrocities of the past century is how many are forgotten. A denial campaign—one guided by the Reagan administration, in support of El Salvador’s anti-Communist government—swept El Mozote under the rug for nearly a decade. Several years ago, composer David T. Little…