“I THINK we are on track” in Afghanistan, Defense Secretary Leon Panetta cheerily told reporters on Friday. The Pentagon chief was celebrating the withdrawal of the last of the 33,000 “surge” troops sent to the country by President Obama in 2009, a pullout which — not at all by coincidence — was completed just weeks before the November election. Mr. Panetta was largely right in saying that the surge “accomplished its objectives” of breaking the momentum of the Taliban and buying time for the expansion and training of the Afghan army.

But after a week in which most joint operations between coalition and Afghan troops were suspended, U.S. strategy in Afghanistan is anything but “on track.” In fact, it may be more imperiled than at any other time in Mr. Obama’s presidency.

The halting of contacts below the battalion level between coalition and Afghan forces was ordered last Sunday, after four more American troops were shot and killed by Afghan police. These “green-on-blue” killings, which now account for 51 coalition fatalities this year, are being compared by senior U.S. officials and members of Congress to the Vietnam War’s Tet Offensive, because of the devastating effect on troop morale and the already weak domestic support for the war.

The parallel is hyperbolic — but the suspension of “partnering” will cripple NATO’s strategy if it is prolonged. Up to 80 percent of combat operations in recent months were joint operations between Western and Afghan forces, often at the company level. Without coalition support, many Afghan units may be unwilling or incapable of fighting. In the countryside, they will be tempted to surrender or strike truces with Taliban forces.

Coalition officials say that only about a quarter of the inside killings can be attributed directly or indirectly to the Taliban. Still, the infiltrations represent a clever shift of strategy by an enemy that was driven by the surge from most of its strongholds in southern Afghanistan.

NATO must be equally inventive in finding ways to respond — just as it has reacted to other chronic threats, like roadside bombs. Some obvious steps are already underway, like a rescreening of personnel in the Afghan army and police, which have grown from 100,000 to 350,000 since 2007. More NATO soldiers are being assigned to “guardian angel” patrol duties. When events occur that may inflame Afghan emotions, such as the current reaction to an anti-Muslim film, commanders may be wise to hold back on non-essential operations.

Still, the threat of infiltration and fratricide should not be allowed to break NATO’s strategy of gradually drawing down forces between now and the end of 2014 while continuing to train and mentor Afghan troops. An accelerated withdrawal or the abandonment of the Afghan army would only lead to far greater problems. U.S. strategy on Afghanistan should remain “on track,” as Mr. Panetta put it — even if it is off the rails at the moment.