Mr. Buttigieg said that could be a reflection of wariness over “endless war,” and the confusion generated by a conflict in which the American goals have often seemed to shift. He recalled that when he exercised at the gym at the headquarters of the international security force in Kabul, he would stare at a large graphic that had “eight lines of effort on it, and it was very hard to understand what the scope of the mission was.”

Yet even the central goal of protecting the American homeland from another attack, a staple of John Kerry’s run for the presidency in 2004 and Mr. Obama’s in 2008, barely gets a mention now. Mr. Kerry told The New York Times in 2004 that any effective Afghanistan plan “requires destroying terrorists. And I’m committed to doing that. But I think I have a better way of doing it.”

Five years later, Mr. Obama overruled warnings from his ambassador in Kabul that his administration’s plan to surge troops into the country, then depend on the Afghan government to defend itself, would probably not work. Speaking at West Point, Mr. Obama said he had “determined that it is in our vital national interest to send an additional 30,000 U.S. troops to Afghanistan. After 18 months, our troops will begin to come home.” Quietly, his White House set up an “Afghan Good Enough” committee to find an exit.

The current presidential candidates seem uninterested in revisiting those decisions. Instead, they race to reassure voters about how quickly they would bring the remaining 14,000 troops home.

To some Democrats who devoted years to stabilizing Afghanistan, the candidates are losing an opportunity to take on Mr. Trump for what they view as a feckless foreign policy, one in which the president is trying to rush through a bare-bones peace agreement with the Taliban so that he can announce major troop withdrawals before the 2020 election.

“It would be unworthy of the U.S. to leave the Afghan people and government to the mercies of the Taliban in an unequal agreement,” R. Nicholas Burns, a former under secretary of state for policy and now an adviser to Mr. Biden, said over the weekend.