Kosovo’s secession from Serbia, while winning applause in Washington and Europe, is a grave defeat for international law and international order.

Kosovo’s independence did not come about unassisted. This summer, America and Europe went to the United Nations Security Council seeking authorization and legitimacy for partitioning Serbia between the Serbs and secessionist Albanians. The Security Council refused to authorize the carve-up of an existing country along ethnic lines — consistent with the international rejection of dividing Iraq into three ethnic states.

Having failed to win Security Council approval, America and Europe simply indicated to the Kosovars that they could announce independence anyway. Serbia of course, cannot resist the secession militarily, as Kosovo has been held by NATO troops since 1999. Thus the North Atlantic Treaty Organization is providing the vital military cover for Kosovo’s independence. Kosovo did not simply tear itself away from Serbia. It was conquered from the Serbs by NATO and the West, and handed over.

So America makes a pitch to the Security Council, gets rejected, and then does what it was going to do anyway. Sound familiar?

The defiance of international law dates back to NATO’s original intervention in Kosovo in 1999. Responding to Serb abuses against the ethnic Albanians in the region, NATO launched a campaign of aerial bombardment against Serbia. The Security Council also refused to authorize this action, and international lawyers widely agree that despite its humanitarian motives, NATO’s war was completely illegal. Further, since even small military casualties could spoil public support of a poorly-understood war, the bombing was carried out from high altitudes to minimize military causalities at the expense of greater collateral damage.

The Serbian military was finally kicked out of Kosovo, and an incipient ethnic cleansing halted. NATO forces secured the province. On NATO’s watch, the Albanians turned the tables, perpetrating ethnic cleansing on the Serbs, a large proportion of whom were chased out of the province through murders, church burnings, and pogroms. With their security forces barred from the province by NATO, the Serbs, who are a majority in Serbia but a minority in the Kosovo province, were powerless to defend themselves. NATO deserves the credit for stopping the ethnic cleansing of Kosovo’s Albanians, but must bear at least some responsibility for the ethnic cleansing of the Serbs.

The Kosovars invoke the international law principle of self-determination. They constitute 90% of the population in the region, having bolstered the ratio by evicting most of the Serbs, gypsies, and any other minority who lived there.

Yet self-determination is no guarantee of independence. The Tamils in Sri Lanka, the Russians in Transdniester/Moldova, the Kurds in Turkey, Syria, Iraq, and Iran; Uighurs in China, Chechens in Russia, and similar groups in dozens of other countries also predominate in a particular region, and have made massive and violent efforts to win self-determination.

Yet in all these cases, the policy of the West is either indifference or outright opposition to secession. It seems self-determination is a principle of very selective application. Indeed, the Serbs in Kosovo are actually a majority in one well-defined enclave; they themselves wish to secede, but the new Kosovar government will hear nothing of Serb self-determination within Kosovo.

An important ingredient of Kosovo’s success in achieving self-determination seems to be their constant threats of violence. The Kosovar prime minister, a former leader of an armed rebel movement, often warned of “dangers” and “unforeseeable consequences” if the province were not allowed to secede. With 16,000 NATO troops in the area, the last thing Europe wanted was an insurgency that could become a jihad-magnet.

As a result, NATO and America have become parties to the carve-up a sovereign state that they subdued by force. To say that this goes against the core principles of the U.N. Charter is an understatement. For international law, the entire process is a string of humiliations. The Security Council comes out looking like a joke; the right of self-determination looks like it depends on the product of a group’s ruthlessness and proximity to Europe; peacekeepers are hostages; and sovereignty is trumped by the threat of terror.

Mr. Kontorovich is a professor at Northwestern University Law School, specializing in international and constitutional law.