On Patrol in Bosnia.(National Guard on peacekeeping duty in Bosnia)

Lt. Matt Stapleton is a long way from his weekend combat training. On an unseasonably warm January morning, the North Carolina National Guardsman--a volunteer part-time soldier back home in the States--is bouncing in a Humvee through Bratunac, a town in eastern Bosnia whose entire Muslim population was murdered or driven out during the war. At a construction site outside town, Stapleton approaches Aljo Mehmedovic, 60, who returned last month to the ruins of his home just yards from a mass grave where seven neighbors were recently exhumed.

Mehmedovic has begun rebuilding his house with donated materials--under the watchful eyes of American troops. "The windows and doors just arrived," he tells Stapleton, as the lanky commander nods in approval. "The coal came last week. Everything is good." Climbing back into the Humvee, Stapleton takes a bite of a sugar doughnut and a sip of coffee. "So few people are coming back to this area," Stapleton tells a visitor. "The only reason they feel safe at all is because we're here."

The question is, how long will they remain? The Bush administration has made no secret of its desire to withdraw U.S. peacekeepers from the Balkans and let European troops take over the job. In December the new head of the U.S. National Security Council, Condoleezza Rice, assured Wolfgang Petrisch, Bosnia's chief international administrator, that the Bush team would not move hastily; they would phase out American involvement--and only in consultation with European partners. But even the possibility of U.S. disengagement is alarming to many Bosnians and others in the Balkans. They say that the country's fragile peace could collapse without U.S. support. American troops on the ground agree. Abandoning Bosnia, they say, would invite violence by ethnic extremists on all sides, and gravely damage America's credibility with its allies. "We can't just say, 'Let's divorce ourselves from NATO'," says Maj. Gen. Walter L. Sharp, commander of the U.S. forces in Bosnia.

Five years after the Dayton peace accord, Bosnia has made some progress toward stability. Unlike Kosovo, Bosnia has functioning local governments and has kept a lid on ethnic violence. But it is still divided into two mutually suspicious entities, the Muslim-Croat Federation and the Republic of Srpska, and all three ethnic groups are effectively led by the same nationalist parties that held power during the war. Cities and towns remain sharply split along ethnic lines; almost all schools are segregated, and 800,000 internally displaced people still haven't returned home. Suspected war criminals hold key positions of power in the police and local government, especially in the Republic of Srpska. "Bosnia is a thin facade held up by a framework of international support," charges James Lyon, director of the International Crisis Group, a human-rights watchdog organization, in Sarajevo. "If we pull out tomorrow, everything would collapse."

American peacekeepers say they're working hard to prevent that collapse. The United States now has 3,900 troops in Bosnia, down from 18,000 five years ago, all based in the northeast sector of the country. The soldiers conduct demining operations (13 Bosnians were killed by mines last year), inspect weapons-storage sites and run a program of voluntary arms surrenders among the civilian population known as Operation Harvest. They also monitor the International Police Task Force, a United Nations-supervised outfit whose ...