As late as Sept. 25, President Bush may still have believed the campaign-era mantra that he repeated that day, that the United States is "not into nation-building."

But when the first American bombs fell on Afghanistan Oct. 7, Washington took on a strong and well-nigh inescapable responsibility for the future of that war-ravaged land.

That is so for two linked reasons. The first is the need to craft a viable "exit strategy" for the US military from its current engagements in and around Afghanistan. The stated aims of the current American-led campaign are, after all, to wipe out the Al Qaeda network, and to make Afghanistan's Taliban regime "pay a price" for having sheltered Al Qaeda for so long. Realizing those aims militarily, at this point, probably requires ousting the Taliban from its seat of power, though members of the administration have seemed wary of spelling that out in so many words.

But Washington cannot simply overthrow the Taliban regime, and then wash its hands of any responsibility for what comes after. There's no guarantee, after all, that a successor regime would be any better than the Taliban - even if it came to power on the backs of the US military, and mouthing pro-American slogans. If the US military is to make a victorious exit from the engagement it has embarked on, then US diplomacy needs to work overtime now, planning a viable, pluralistic political order for Afghanistan's future.

Recent decades are littered with examples of US-led deployments in which this vital work of diplomatic planning was neglected: Lebanon 1982-93, Iraq 1990-91, Somalia 1993-94, Bosnia since 1992, and Kosovo since 1999. In all these cases, shoddy political planning for the post-deployment left a US-led force without any possibility of a smooth and successful exit. Those that did not end in debacle, left the US with difficult, draining entanglements in hope-starved distant lands. Not something to aim for in Afghanistan.

The second reason that Americans now have a strong responsibility for Afghan nation-building is our longer-term interest in preventing the recurrence of conditions in which future terrorist networks might take root. Some call this "draining the swamp." I bridle at comparing any fellow humans, however hate-filled, with insects like mosquitoes. But still, the goal of helping the Afghans to clean up their political environment and build themselves a solid stake in a stable future is certainly a good one. Failed and badly failing states like the Afghanistan of the past decade are certainly a menace to their own citizens. But they also - through the spread of terrorism and other hazards - pose a clear and present danger to the rest of us as well.

Personally, I'm sorry that the administration used military force. Al Qaeda could have been pursued with other means, and warfare always ends up harming civilians. But now that force - war - has been initiated, I don't see how Washington can avoid taking responsibility for how this whole unpredictable business of military thrust and counter-thrust is brought to an end. What sort of successor regime should the US aim for, and how should we go about trying to achieve it?

A firm basis of respect for pluralism will be key, if Afghanistan's much-abused people are ever to enjoy peace. The 27 million Afghans come from a rainbow of different language groups, ethnicities, and types of Islamic belief. The Taliban, most of whom are ethnic Pushtuns, have tried for years to quash that diversity. Many of our present "friends" in the Northern Alliance have acquired disturbing reputations for their own pursuit of intergroup hatred and violence. The best hope for a unifying and pro-pluralism figure may be the aging, long-exiled king, Mohammad Zahir Shah; he could end up playing a role like that of King Juan Carlos in post-dictatorship Spain.

The US need not - and should not - pursue its diplomatic planning for Afghanistan all alone. The UN Security Council has been closely involved in Afghan political affairs for many years. Afghanistan's neighbors, including very-vulnerable Pakistan, all need to be involved somehow in the planning, both to ensure their future noninterference, and to be reassured that their own legitimate interests will not be threatened by the Afghanistan that emerges.

Finally, all this nation-building - all this peace-building - will need considerable resources. As Germany's past century shows, rebuilding physical and sociopolitical infrastructure is by far the best way to build up a people's stake in long-term stability. Sending food to stave off the Afghans' immediate risk of famine, as the US has begun to do, is a good (but tiny) start. Long-term peace-building is a much bigger proposition.

Maybe we could use some of the billions of bucks the administration has been planning to spend on the crazy, NATO-busting project of a national missile defense. For if Sept. 11 showed us anything, it was that investing in relationships, and the human and political bases of peace, serve our security much, much more than go-it-alone technological wizardry ever could.

* Helena Cobban is a veteran journalist and author of five books on international issues.