After 18 years, is this peace or just a way to slip away?

President Donald Trump has left no doubt that his first priority in Afghanistan is a peace treaty that would enable him to claim that he is fulfilling his vow to withdraw U.S. troops.

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

And that creates an enormous risk for Trump and for Afghanistan: that, like President Richard Nixon’s peace deal with North Vietnam in January 1973, the accord signed Saturday will speed a U.S. exit and do little to stabilize an allied government. In the case of Vietnam, it took two years for the “decent interval,” in Henry 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? President and Foreign Policy From F.D.R. to Trump,” examines the Vietnam precedent. Three successive U.S. presidents have promised victory in Afghanistan, even if they each defined it differently. Each experienced failures of political will and on the battlefield.

President George W. Bush began the Afghan war to hunt down Osama bin Laden in the months after the Sept. 11, 2001, terrorist attacks. Yet he soon turned his attention to Iraq and, despite denials by the White House at the time, bled resources from the Afghanistan effort to fuel his next war.

President Barack Obama called Iraq a strategic mistake but pledged that the U.S. would not lose the “good war” in Afghanistan. Yet his brief “surge” failed to strike a decisive blow. Strategy soon was turned over to a small group inside his White House that was aptly nicknamed the “Afghan Good Enough” committee.

Trump long has lamented the cost of “endless wars,” and by the time he took up direct negotiations with the Taliban, he knew U.S. voters were interested mostly in one thing: ending participation in a war that has now dragged on for more than 18 years, its objectives always shifting.

When historians look back at the moment, they may well conclude that Washington ended up much like other great powers that entered Afghanistan’s rugged mountains and punishing deserts: frustrated, immobilized, no longer willing to bear the huge costs. The British retreated in 1842 after suffering 4,500 killed, amid massacres that preceded the invention of the roadside bomb. They gave up their sovereignty over the country in 1919 in another retreat that heralded the beginning of the unwinding of an empire.

The Soviet Union abandoned its decadelong effort to control the country in 1989, months before the fall of the Berlin Wall marked the collapse of the communist superpower. That led to the chaos and power vacuums that bin Laden exploited and that the United States vowed it would never again allow to fester.

The U.s.-led attack began Oct. 7, 2001, with the name Operation Enduring Freedom. Bush, in an address to the nation from the Treaty Room of the White House, promised to “win this conflict by the patient accumulation of successes, by meeting a series of challenges with determination and will and purpose.”

After the destruction of the twin towers of the World Trade Center, a plane that hit the Pentagon and another that crashed in a field in Pennsylvania, the country was behind him. Though a few warned of the dangers of entering the “graveyard of empires,” it seemed more a war of retribution and justice seeking than an effort at nation-building.

Yet inevitably, mission creep set in.

After bin Laden was hunted down in Pakistan in May 2011, and with al-qaida a much-diminished threat, politicians struggled to explain what U.S. troops were fighting to accomplish. More than 2,400 service members have died in combat since the invasion, according to the website icasualties.org. Neither Obama nor Trump could make a plausible case that after nearly two decades the United States had much of a role to play other than prop up a weak democracy.

For a while, at least, that role seems likely to continue.

The accord signed Saturday — with Secretary of State Mike Pompeo overseeing the moment but not actually signing it himself — initially will bring down U.S. troop levels to about 8,600 from about 12,000 now. That is almost exactly where they were three years ago at the end of Obama’s term. That is the minimum number of Special Operations forces, intelligence officers and support and security personnel that the Pentagon and CIA believe are necessary to hold the capital, Kabul; battle militants of Islamic State; and advise an Afghan military that remains, at best, a fractured, inconsistent fighting force after close to two decades of training and billions of dollars in U.S. and NATO investment.

If the Afghan government can reach its own accord with the Taliban — in a so-called intra-afghan process that is now supposed to begin — the U.S. troop levels may drop further, officials said. But just as the South Vietnamese were not part of the Paris peace talks a half-century ago, the Afghan government, whose survival is at stake, was excluded from the long negotiations with the Taliban.

That explains why President Ashraf Ghani was so suspicious of, and often furious with, the negotiating process — and made clear during the Munich Security Conference this year that he had little confidence that the next step in the accord would come to pass.

It was telling that he was not at the signing ceremony in Doha, Qatar; instead, Defense Secretary Mark Esper was dispatched to Kabul for a separate ceremony with Ghani and the secretary-general of NATO to signal the allied support of the government.

The only party missing was the Taliban, whose leaders on Friday refused to meet with an informal Afghan negotiating team.

It is the Afghan’s concern that they will be abandoned that led more than 20 Republicans and Democrats to send a letter this past week to Pompeo and Esper warning that “the Taliban is not a de facto counterterrorism partner, and pretending that they are ignores their longtime jihadist mission and actions.”

It added, “They have never publicly renounced al-qaida or turned over al-qaida leaders living in their safe havens” or “apologized for harboring the terrorists who carried out the September 11 attacks.”