A decade ago, Bosnia was torn apart by ethnic cleansing and facing its demise as a single country. After much hesitation, the United States stepped in decisively with the Dayton Accords,which kept the country whole by, paradoxically, dividing it into ethnic federations, even allowing Muslims, Croats and Serbs to retain separate armies. With the help of American and other forces, Bosnians have lived a decade in relative peace and are now slowly strengthening their common central government, including disbanding those separate armies last year.

Now the Bush administration, despite its profound strategic misjudgments in Iraq, has a similar opportunity. To seize it, however, America must get beyond the present false choice between “staying the course” and “bringing the troops home now” and choose a third way that would wind down our military presence responsibly while preventing chaos and preserving our key security goals.

The idea, as in Bosnia, is to maintain a united Iraq by decentralizing it, giving each ethno-religious group — Kurd, Sunni Arab and Shiite Arab — room to run its own affairs, while leaving the central government in charge of common interests. We could drive this in place with irresistible sweeteners for the Sunnis to join in, a plan designed by the military for withdrawing and redeploying American forces, and a regional nonaggression pact.

It is increasingly clear that President Bush does not have a strategy for victory in Iraq. Rather, he hopes to prevent defeat and pass the problem along to his successor. Meanwhile, the frustration of Americans is mounting so fast that Congress might end up mandating a rapid pullout, even at the risk of precipitating chaos and a civil war that becomes a regional war.

As long as American troops are in Iraq in significant numbers, the insurgents can’t win and we can’t lose. But intercommunal violence has surpassed the insurgency as the main security threat. Militias rule swathes of Iraq and death squads kill dozens daily. Sectarian cleansing has recently forced tens of thousands from their homes. On top of this, President Bush did not request additional reconstruction assistance and is slashing funds for groups promoting democracy.

Iraq’s new government of national unity will not stop the deterioration. Iraqis have had three such governments in the last three years, each with Sunnis in key posts, without noticeable effect. The alternative path out of this terrible trap has five elements.

The first is to establish three largely autonomous regions with a viable central government in Baghdad. The Kurdish, Sunni and Shiite regions would each be responsible for their own domestic laws, administration and internal security. The central government would control border defense, foreign affairs and oil revenues. Baghdad would become a federal zone, while densely populated areas of mixed populations would receive both multisectarian and international police protection.

Decentralization is hardly as radical as it may seem: the Iraqi Constitution, in fact, already provides for a federal structure and a procedure for provinces to combine into regional governments.

Besides, things are already heading toward partition: increasingly, each community supports federalism, if only as a last resort. The Sunnis, who until recently believed they would retake power in Iraq, are beginning to recognize that they won’t and don’t want to live in a Shiite-controlled, highly centralized state with laws enforced by sectarian militias. The Shiites know they can dominate the government, but they can’t defeat a Sunni insurrection. The Kurds will not give up their 15-year-old autonomy.

Some will say moving toward strong regionalism would ignite sectarian cleansing. But that’s exactly what is going on already, in ever-bigger waves. Others will argue that it would lead to partition. But a breakup is already under way. As it was in Bosnia, a strong federal system is a viable means to prevent both perils in Iraq.

The second element would be to entice the Sunnis into joining the federal system with an offer they couldn’t refuse. To begin with, running their own region should be far preferable to the alternatives: being dominated by Kurds and Shiites in a central government or being the main victims of a civil war. But they also have to be given money to make their oil-poor region viable. The Constitution must be amended to guarantee Sunni areas 20 percent (approximately their proportion of the population) of all revenues.

The third component would be to ensure the protection of the rights of women and ethno-religious minorities by increasing American aid to Iraq but tying it to respect for those rights. Such protections will be difficult, especially in the Shiite-controlled south, but Washington has to be clear that widespread violations will stop the cash flow.

Fourth, the president must direct the military to design a plan for withdrawing and redeploying our troops from Iraq by 2008 (while providing for a small but effective residual force to combat terrorists and keep the neighbors honest). We must avoid a precipitous withdrawal that would lead to a national meltdown , but we also can’t have a substantial long-term American military presence. That would do terrible damage to our armed forces, break American and Iraqi public support for the mission and leave Iraqis without any incentive to shape up.

Fifth, under an international or United Nations umbrella, we should convene a regional conference to pledge respect for Iraq’s borders and its federal system. For all that Iraq’s neighbors might gain by picking at its pieces, each faces the greater danger of a regional war. A “contact group” of major powers would be set up to lean on neighbors to comply with the deal.

Mr. Bush has spent three years in a futile effort to establish a strong central government in Baghdad, leaving us without a real political settlement, with a deteriorating security situation — and with nothing but the most difficult policy choices. The five-point alternative plan offers a plausible path to that core political settlement among Iraqis, along with the economic, military and diplomatic levers to make the political solution work. It is also a plausible way for Democrats and Republicans alike to protect our basic security interests and honor our country’s sacrifices.

Joseph R. Biden Jr., Democrat of Delaware, is the ranking member of the Senate Foreign Relations Committee. Leslie H. Gelb is the president emeritus of the Council on Foreign Relations.