That massacre unleashed the Mexican Left, encouraged by the victory of the Cuban Revolution. Much of their activism emerged from the Ayotzinapa teachers college, the Escuela Normal Rural Raúl Isidro Burgos. Founded in 1926 and still closely linked to its Marxist roots, the school’s heroes were Marx and Lenin, and in the 1970s Che Guevara. One of its students was the guerrilla leader Lucio Cabañas, who in the late 1960s, together with Genaro Vázquez Rojas, declared war on the Mexican state — with broad social support.

In those days, Cuba provided material support for revolutionary activism in many Latin American countries. But not in Mexico, the only nation in Latin America to rebuff American pressure to break relations with Cuba. In exchange, the Castro regime neither encouraged nor supplied Mexican revolutionaries. President Luis Echeverría (1970-76) opened Mexico’s doors to refugees fleeing military terror in Chile and Argentina, while at the same time he unleashed terror at home (especially in Guerrero) to wipe out leftist guerrilla movements. Guerrero then subsided into an illusory calm, punctuated by scattered but bloody incidents of violence.

An ominous new protagonist entered the picture in force at the turn of the century. Guerrero had always been a center of marijuana production, but the newer, more organized drug cartels found the state an ideal site for their business, with its rugged and often impassable hills and old culture of violence. The citizens of Guerrero, meanwhile, were full of lingering resentment over the Guerra Sucia (Dirty War) of the 1970s — and often quite as poor as people living in the most impoverished zones of Africa. At the same time, Mexico’s nationwide problem of political corruption became especially acute in Guerrero. In many towns, mayors and their colleagues took bribes from the narcos, associated with them, or, as in Iguala, were even themselves the drug lords.

The leftist Party of the Democratic Revolution (PRD), which has governed Guerrero since 2005, has done almost nothing to break the old links between politics and crime. In fact, the mayor of Iguala and the governor of the state had the support of all of the party’s national leaders. The centrist Institutional Revolutionary Party (PRI) won the presidential election of 2012 with the promise of returning peace and security to the people. But the government has been, at the very least, inattentive and inefficient. The army has a major garrison near Iguala but, inexplicably, has done nothing to inhibit the alliance of criminals and politicians.

The state of Guerrero produces 98 percent of the heroin-yielding poppies in Mexico; production has grown exponentially. President Obama recently quoted a report by the United States Drug Enforcement Agency of a “324 percent increase in heroin seizures at the Mexican border between 2009 and 2013.”