Yet it did. On Oct. 2 came the denouement. There was a protest meeting that afternoon in the Plaza of Tlatelolco. The army had been given orders to dissolve the meeting, but their assault ended in a cross-fire between soldiers and the mysterious “Olympic Battalion,” a paramilitary group created by the government posted in nearby buildings.

It was the unarmed students who paid with their lives. The hellish violence went on for hours. No one knows the exact number of dead. There was talk of hundreds. Perhaps there were even less than 100 slain, but the scenes of horror, the arrests and imprisonments, the revelations of torture, would linger in our collective memory down to the present day.

Over the years, various theories about the massacre have been proposed. The C.I.A. seemed to believe in a conspiracy forged in Cuba, a theory that was brandished by Mr. Díaz Ordaz. However, for better or for worse, such an idea was highly unlikely, since Mexico was the only Latin-American country that had refused to break off relations with Castro. In his unpublished memoirs, Mr. Díaz Ordaz asserts that Mexico was “at war.” The students were “the opponents.” After the killings, he would write, in a tone of satisfaction, “They finally obtained their insignificant deaths (sus muertitos).”

The real war unleashed in 1968 Mexico was the struggle for the presidential succession in 1970. As had been the custom since 1929, a Mexican president would designate his successor. Various government secretaries would wage a no-holds-barred struggle to gain presidential favor. Luis Echeverría, secretary of the interior, would finally be the winner because in the eyes of Mr. Díaz Ordaz he showed “more pants” and was “the boldest.” And he also effectively nourished the president’s paranoia.

Perhaps the greatest contribution of ’68 was in behalf of freedom of expression. Although as president, Mr. Echeverría, Mr. Díaz Ordaz’s successor, tried to ingratiate himself with university students through a rhetorical veering to the left, the criticism he kept receiving from the newspaper Excélsior (very much in the spirit of ’68) exasperated him enough that he maneuvered a coup (in July of ’76) against its editor, Julio Scherer.