Now that Hamid Karzai's rival has pulled out from the Afghan presidential race and the runoff cancelled, the stage is set for a last-ditch effort to reverse the declining fortunes of the international mission in that country.

Barack Obama has apparently been waiting for a resolution of the Afghan election fiasco before announcing the result of his review of U.S. policy toward Afghanistan. In the next few days or weeks, he is likely to endorse the recommendations of his hand-picked Afghan commander, General Stanley McChrystal, who has called for a shift in NATO's strategy toward the kind of counterinsurgency approach that worked in Iraq – one that prioritizes the protection of the Afghan population over the killing of insurgents.

Mr. Obama probably will also deploy more U.S. troops, although perhaps not the full 40,000 additional forces that Gen. McChrystal has reportedly requested.

As the White House studied its options, more Americans have called for the withdrawal of U.S. ground troops from Afghanistan. These calls have come not only from the Democrats' liberal base but also from conservative foreign-policy “realists” such as columnist George Will, Harvard's Stephen Walt and the University of Chicago's Robert Pape, who argue that al-Qaeda can be battled from a distance using U.S. commandos, cruise missiles and armed drones.

By all appearances, Mr. Obama will not heed these calls – at least, not yet. Although the costs of continuing the mission are enormous and the risk of failure is real, he likely recognizes that precipitous disengagement poses even greater dangers.

If NATO forces left Afghanistan, the most probable outcome would be an all-out civil war pitting the Taliban and their allies against a remobilized Northern Alliance. The scale of violence would almost certainly dwarf the relatively small-scale guerrilla war now under way.

Any chance of working with more moderate leaders would evaporate, and transnational jihadist groups, still bent on attacking the West, would likely cross over from Pakistan and re-establish themselves in Afghanistan. We've seen this movie before – in the 1990s, including America's reliance on ineffectual cruise-missile strikes from a distance – and it had a terrible ending.

Even more troubling, renewed civil war in Afghanistan could further destabilize Pakistan. Elements of the Taliban have turned their sights on Islamabad. If the Afghan insurgency were to gain greater freedom of operation in Afghanistan, those seeking to overthrow Pakistan's government probably would also grow stronger.

Beyond the threat these groups pose to Pakistan's stability, some have also been involved in terrorist attacks on India, Pakistan's historic rival. (Some jihadist groups were allegedly created by Pakistan's intelligence service for this purpose.) How long would the Indian government stand by, without taking direct military action, if Pakistan-based militants launched more frequent and devastating attacks on India?

To cap it off, Pakistan also has nuclear weapons, as does India. The prospect of Pakistan's nukes falling into the wrong hands is deeply unnerving. We are told these weapons are secure, but how reliable are these assurances? Just last month, militants succeeded in penetrating the Pakistani army's headquarters.

For all these reasons, Mr. Obama will almost certainly conclude that this is not the time to withdraw from Afghanistan. But it is also unlikely that he – or the increasingly war-fatigued American people – will tolerate continued failure much longer.

Gen. McChrystal seems to understand this, too. In a recently leaked report, he offered the following blunt warning: “Failure to gain the initiative and reverse insurgent momentum in the near term (next 12 months) – while Afghan security capacity matures – risks an outcome where defeating the insurgency is no longer possible.”

In other words, the possibility of long-term success in Afghanistan – minimally defined as the existence of an Afghan government capable of maintaining a reasonable degree of security in most areas of the country – is years away, even in the best-case scenario. But it will be impossible to achieve this outcome if, in the short run, NATO can't at least halt the progressive worsening of conditions in Afghanistan. This is the litmus test for Gen. McChrystal's approach: Can his strategy, within one or two years, stem the steady slide toward defeat?

If, by then, the insurgency is still strengthening and the Afghan government is still weakening, it is difficult to imagine maintaining a massive U.S. military force in Afghanistan, because there will be little remaining hope of building up an Afghan government with the capacity to provide for its own security.

But we are not there yet. Nor is a turnaround impossible. Conditions in Iraq were arguably bleaker before the American “surge” in 2007 than they are in Afghanistan today. And for now, the costs of continuing the mission along the lines recommended by Gen. McChrystal are still outweighed by the risks of withdrawal.

Yet, time is very short. The mission seems to be entering a decisive phase: either the final hurrah before withdrawal, or a dramatic turnaround offering the hope of achieving longer-term stability in Afghanistan.

Roland Paris is director of the Centre for International Policy Studies and associate professor in the Graduate School of Public and International Affairs at the University of Ottawa.