ONE MORE truism of the Kosovo conflict is falling victim to fact. It was often repeated, while the bombing went on, that the deportees would never go home, that experience from Afghanistan to Bosnia proved that such a mass exodus could never be reversed. But since the bombing ended, NATO's problem if anything has been the opposite: Kosovo's population has been streaming home faster than NATO thinks safe.

For a sizable but unknown number, however, return still is not possible. Many Kosovars, particularly men, are being held captive inside Serbia. Serbian officials have told the Red Cross that they are holding more than 2,000; according to other estimates, the number may be 5,000 or even greater. Some have been prisoners for many months; Slobodan Milosevic agreed last October to release these detainees but reneged on his promise. Others were studying or working in Belgrade when NATO began bombing in March and were promptly rounded up. And many are believed to have been trucked across the border as Serbian forces retreated earlier this month. Given the Serbian torture chambers NATO troops have discovered in Kosovo, no one can feel easy about the detainees' condition.

A few days ago Serbian authorities released 166 of these prisoners, gaunt but alive. The rationale for the small release is as unclear as the motivation for keeping the larger number. Mr. Milosevic seems to have planned the destruction of ethnic Albanian Kosovo as a working society. The imprisonment of some Kosovo men, like the killings and forced expulsion of others, was undoubtedly part of this plan. But NATO's bombing campaign forced him to relinquish political control, and it's not clear why he's holding on to his prisoners. Spite, a desire to weaken Kosovo's leadership as it attempts to rebuild, a belief that the prisoners may be a useful bargaining chip -- these are all possibilities.

The United States and its allies erred in not including prisoner return as a plank of the peace agreement. They can compensate now by putting it at the top of their policy priorities. The war is not over until Mr. Milosevic accounts for the Kosovars he has kidnapped and allows them to go home.