The arithmetic of human hatred computes this way in Kosovo's ancient town of Pec: Before the war one-fifth of Pec's population was Serb. Today four-fifths of the town's houses are charred wrecks.

Slobodan Milosevic did not set fire to each of those houses in Pec. Individual Serbs did. They did so willingly and often with murderous glee, according to the survivors' accounts now being gathered by journalists, war crimes investigators and NATO peacekeepers.

Most of the Serbs who systematically destroyed their neighbors' homes are fleeing. Their houses began to burn as soon as the first ethnic Albanian refugees came home. As a result the Serb population of Pec is heading rapidly toward a new percentage: zero.

"If we have to protect every Serb here, all the armies in Europe will not be enough," an Italian lieutenant told a correspondent for the French newspaper Le Monde earlier this week, undermining his commanders' appeals for Serbs to stay in Kosovo and just get along with those they victimized for a decade.

The lieutenant is more realistic than are his leaders about the grim destiny the Serbs have sown and now reap for themselves in Kosovo and inside Serbia. The Serbs fleeing the province are not the victims of reverse ethnic cleansing, or of the imposition of collective guilt, as apologists frequently claim. They are the victims of their community's support for or acquiescence in Milosevic's war.

As individuals they may be guiltless. But the attempted murder of Kosovo was carried out willingly by Serbs as a national act. Conceived and implemented by their national leaders -- who remain in office -- the campaign to reduce the Kosovar Albanians to absolute, final submission to Serb political will has summoned forth failure and its consequences for the Serbs.

The thought is not that NATO troops and the world at large should stand aside if the Kosovars want to even scores on suddenly out-gunned Serb civilians. That would only guarantee an unending and ever-bloodier Balkan war. Nor is the spirit one of vengeful punishment on Serbia, which will soon be appealing for outside aid to get through the winter. Purposeful vengeance also insults the future and guarantees permanent conflict.

NATO and the United Nations will be required to cooperate in running Kosovo as a heavily policed international protectorate for the immediate future. Their member states have another important task: They must avoid helping the Serbs escape the consequences of the actions they have allowed their leaders to take in their name.

The Serbs cannot be allowed to find ways to continue to ignore the horrors of Kosovo, or to arrange those horrors into a comfortable version that fits with their widespread complex of persecution. The key struggle in Kosovo and Serbia at this moment is not diplomatic or military. It is moral and psychological.

For the outside world, that struggle involves at a minimum doing nothing that would delay the national reassessment the Serbs need to make of themselves and their society. The war -- and the lack of significant dissent it created in Serbia -- show that their political framework is based not on police-state terror but on voluntary Stalinism.

Censorship and propaganda do not fully explain general Serb indifference to Kosovo's Calvary. There were no significant differences in public reaction by Serbs with access to Western accounts and live broadcasts and those without such access. Individuals felt free to criticize Milosevic to Western journalists, but Serbia's political parties did not debate the wisdom or morality of the war. Minds were shut and hearts were hardened in ways that are frankly hard to imagine at the end of the 20th century.

That is why it is important for all nations to find ways to maintain the powerful display of moral outrage the regime's actions have earned. Only such a display can exert pressure on Belgrade to grasp the nettle of change.

Britain's political leaders seem to understand this most clearly, missing no opportunity to stress the enormity of what has happened. Foreign Minister Robin Cook personally visited an atrocity site in Kosovo this month. Prime Minister Tony Blair has forcefully rejected French and German calls to rebuild bombed electricity power plants and water pumps near Belgrade, even if Milosevic remains in power.

The power plants remain divisive within the alliance, as Bonn and Paris argue that electricity is a humanitarian need. During the war, France blocked U.S. efforts to put the plants out of action early, yielding only in mid-May to avoid a NATO debate on using ground troops. Milosevic then accepted peace terms he had rejected for two months.

The Serbs must not be left alone to suffer and embitter themselves further. But the contact the West has with them now must be based on change, not on reconstruction that would help them avoid the painful consequences of their own evil. They can have blood-stained leaders, or they can have Western help for power plants. They cannot have both.