UPPSALA, Sweden -- The sudden fall of Kabul may be a great victory for the U.S.-led coalition, but it presents new threats, which may not be easy to handle.

In humanitarian terms, this is good news. Supplies now can reach most of Afghanistan's population before winter.

The crumbling of Taliban power happened more quickly than expected, perhaps too quickly. Looting and summary executions have been reported in Mazar-i-Sharif, highlighting the need for a rapid introduction of a peacekeeping force. The Taliban always knew that the time would come to take the war back to its heartland around Kandahar.

The United States had wisely urged the Northern Alliance to stay out of Kabul for several reasons: fears that alliance commanders might indulge in punitive massacres against pro-Taliban Pushtuns, Pakistan's apprehension with the alliance and the risk that it would establish itself in power, undermining the quest for a broad-based government and leading to a continued civil war.

Given a human-rights record as bad as that of the Taliban, there is a risk of payback against Pushtun civilians who may have supported Taliban rule. Because the United States paved the way for the alliance to take over, the U.S. could be held responsible.

This could effectively end hopes of a significant component of Pushtuns--close to half of Afghanistan's population--joining the coalition. Without that, continuing the war effort against the Taliban and Osama bin Laden would be problematic and the future government would lack legitimacy.

Even more alarmingly, if the war takes on a clear ethnic character, the already tense situation among Pakistan's 20 million Pushtuns may become uncontrollable, and the U.S. might lose this key coalition ally.

It is imperative to act fast. The United States must take the lead in assembling an international peacekeeping mission to Afghanistan, which should have a strong Muslim component, perhaps from Turkey, Jordan and Indonesia. A forerunner to this larger force needs to be dispatched to Kabul within days, not weeks.

Such a peacekeeping force would reduce the risk of the Northern Alliance seeking to cling to power by itself. As it represents mainly the minority peoples of the north, the alliance should not be allowed to spoil the plans to set up the first broad-based government Afghanistan has seen in decades.

No government in Afghanistan can work without a significant Pushtun component--despite the fact that Russian President Vladimir V. Putin has encouraged the alliance to take over government by itself.

The Taliban retreat does not mean that the war has been won, but merely that it is taking on what probably will be a nastier character: a guerrilla war in Taliban country, the southern mountains.

The U.S. needs allies in the south, where the Northern Alliance will be of no help. The availability of Pushtun allies largely will depend on whether the U.S. can rein in the Northern Alliance in the days to come.

If the Taliban can keep its ranks together, it will fight the United States the way the Soviet Union was fought in the 1980s, with ambushes, lightning raids and other guerrilla tactics.

As was the case in Vietnam and as Moscow learned in Afghanistan and in Chechnya, the problem is that as long as the Taliban does not lose, it will be winning.

And as long as it doesn't lose, Bin Laden has a good chance to stay alive and well.