Mr. Kissinger responded, “If there are things that have to be done, you should do them quickly,” an apparent warning that a new American Congress might cut off aid if it thought the Argentine government was engaging in systemic human rights abuses.

The American ambassador in Buenos Aires soon reported to Washington that the Argentine government had interpreted Mr. Kissinger’s words as a “green light” to continue its brutal tactics against leftist guerrillas, political dissidents and suspected socialists.

Just how much the American government knew about Argentina’s repressive “Dirty War,” which lasted from 1976 to 1983 — and the extent to which it condoned the abuses — has remained shrouded in secrecy.

When President Obama visits Argentina next week during the 40th anniversary of the coup, he should make a pledge that Washington will more fully reveal its role in a dark chapter of Argentine history. Military officials abducted thousands of civilians during this period. Hundreds of babies, stolen from Argentines who were arbitrarily detained, were raised by military families.

Human rights groups in Argentina have long sought access to classified American intelligence and diplomatic records, hoping that they will shed new light on the abuses and the fate of missing Argentines. The Argentine government itself has formally asked for declassification. “There is absolutely no doubt that the release of these records on repression in Argentina would reveal substantive information on the years of repression and advance the cause of truth and justice in that country,” said Peter Kornbluh, an analyst at the National Security Archive who specializes in Latin America.

In 2002, Washington partly declassified roughly 4,700 State Department records from the Dirty War period. Those documents have aided judicial proceedings and added to a historical record. But much of that record remains obscured.

Declassifying a more extensive set of documents would also bring into sharper focus a shameful period of American foreign policy, during which Washington condoned and in some instances supported the brutal tactics of right-wing governments in the region. It is time for the American government to do what it still can to help bring the guilty to justice and give the victims’ families some of the answers they seek.