After 18 years, is this Afghan peace or just a way out?

But a parade of his former national security aides say he is far less interested in an actual Afghan peace.

And that creates an enormous risk for Mr. Trump and for Afghanistan: that, like President Richard M. Nixon’s peace deal with North Vietnam in January 1973, the accord signed Saturday will speed an American exit and do little to stabilize an allied government. In the case of Vietnam, it took two years for the “decent interval,” in Henry A. Kissinger’s famous phrase, to expire and for the South Vietnamese government to be overrun.

“Trump would not be the first president to exaggerate the meaning of a truce in an election year,” said Joseph Nye, an emeritus professor at Harvard whose newest book, “Do Morals Matter? Presidents and Foreign Policy From F.D.R. to Trump,” examines the Vietnam precedent.

In the heat of the 1972 election, Mr. Nye notes, “Nixon made great claims about an imminent peace in Vietnam,” and it was only after his re-election — and his resignation — that the image of a frantic helicopter evacuation from Saigon came to mark the failure of a long, costly American experiment.