IN DESCRIBING the U.S. mission in Afghanistan after meeting President Hamid Karzai last week, President Obama said that “it was in our national security interest to have a stable, sovereign Afghanistan that was a responsible international actor, that was in partnership with us, that . . . [had] its own security capacity and [was] on a path that was more likely to achieve prosperity and peace for its own people.” What he didn’t say was that his administration is well on its way to abandoning that interest.

To preserve a stable Afghanistan after 2014, the United States and its allies would have to leave behind sufficient forces to enable the Afghan army — which currently boasts only one brigade able to act independently — to operate effectively enough to prevent the Taliban from retaking the southern and eastern territories it has been driven out of since 2009. Such a resurgence would likely plunge the country back into the civil war that raged during the 1990s and allowed al-Qaeda to create a haven. Particularly vital are U.S. air support, intelligence, mine-clearing and medevac personnel that give the Afghan army mobility and allow it to stay on the offensive.

Just eight months ago, U.S. and NATO military officials were anticipating that it would take an international force of around 30,000 soldiers, including 20,000 Americans, to accomplish that mission while also continuing to train the Afghan forces and carrying out counterterrorism missions against al-Qaeda. Since then, however, the White House has quietly changed course, overruling the generals and insisting that plans be drawn up for a far smaller contingent. Mr. Obama’s civilian aides are reported to be pushing for 3,000 or less; last week, a National Security Council spokesman told reporters a “zero option” would also be considered.

It seems fairly clear that, with a post-2014 force of 3,000 or fewer, the United States would have to scrap the enabling mission for the Afghan army. It would have barely enough personnel to carry out its own strikes against al-Qaeda. It’s clear, too, that without U.S. support Afghan forces would almost certainly lose substantial ground to the Taliban — losses that U.S. policy would essentially concede.

But Mr. Obama has not been clear. Though he appears to be considering options for a future U.S. presence that would make meaningful training and assistance to Afghan forces impossible, he has not stated, much less justified, an intention to abandon that mission. On the contrary: On Friday he defined the “number one” U.S. goal as “to train, assist and advise Afghan forces so that they can maintain their own security.”

Mr. Karzai, for his part, said that U.S. troop “numbers are not going to make a difference to the situation in Afghanistan. It’s the broader relationship that will make a difference to Afghanistan and, beyond, in the region.” In fact, as the Afghan president well knows, the numbers will reflect Mr. Obama’s commitment to the relationship — and the signals coming from the White House are not promising.