US Defense Secretary Chuck Hagel addresses on December 7, 2014 US troops at the Gamberi forward operating base in the eastern Afghan Laghman province. Hagel said on December 6 during a visit to Kabul that an additional 1,000 US troops would remain in Afghanistan next year to meet a temporary shortfall in NATO forces. US President Barack Obama approved the move despite an earlier plan to limit the US force to a maximum of 9,800 troops in 2015. (Mark Wilson/AFP/Getty Images)

NOT MUCH attention was paid in Washington to the formal end Monday of U.S. combat operations in Afghanistan. The 13-year-old war is something most Americans, led by their president, are eager to put behind them. That’s unfortunate, because far from fading away, the fighting in Afghanistan is intensifying — and so is the threat it poses to everything that the U.S.-led coalition accomplished.

Over several months, attacks by the Taliban have steadily escalated, including in the once-relatively secure capital. International aid groups are pulling their staff out of Kabul after a wave of bombings and assaults on foreigners’ compounds, while many educated and affluent Afghans who returned from exile to invest in the country are leaving again. The new government under President Ashraf Ghani promises an improvement on the corruption-plagued and ineffective administration of Hamid Karzai, but it is struggling to overcome its internal divisions and appoint a cabinet.

The Afghan army built by the United States and its allies at huge expense is under enormous stress. More than 5,000 soldiers and police have been killed this year — more than the total number of U.S. and allied deaths since 2001. Lt. Gen. Joseph Anderson, who departed the country after overseeing the final year of combat operations, told the New York Times that the Afghan casualties, as well as a high desertion rate, are not sustainable. His assessment of the ability of government forces to hold off the Taliban was cautious: “I don’t know if I’m pessimistic or optimistic,” he said.

The deteriorating situation can only be exacerbated by the rigid timetable imposed by President Obama for ending combat missions and withdrawing the remaining U.S. troops. NATO forces have fallen from 130,000 in 2013 to 12,000, including 10,800 Americans. Mr. Obama has ordered that the U.S. force be cut to 5,500 by the end of 2015 and reduced to a few hundred by the time his administration ends a year later. It’s a political timetable that suits the president’s legacy aspirations but bears no relation to the military situation.

Not surprisingly, Mr. Ghani is said to be lobbying senior U.S. officials to slow the further drawdown of forces and retain a larger number of forces in 2016. The Wall Street Journal reported that, while the new president hasn’t made a formal request, he may do so when he visits Washington early next year. Mr. Obama has shown a bit of flexibility. The number of U.S. troops was temporarily increased by 1,000 this month to compensate for the slowness of allies in committing forces, and rules of engagement for 2015 were tailored to allow some combat missions against Taliban or al-Qaeda targets as well as air support for Afghan forces.

Mr. Obama, however, should go further. The instability in Afghanistan, and probably the Taliban’s heightened aggression, is being driven by uncertainty about whether the United States and its allies will continue to stand behind the government and army and prevent their defeat. The president should make clear that he will not allow the state built since 2001 to crumble — even if that means adjusting his timetable.