The proposed land swap has been lurking in the background since the early days of Kosovo’s independence. But it has gone nowhere in part because the United States and the European Union have adamantly opposed it. True to form, Chancellor Angela Merkel of Germany recently said that “the territorial integrity of the states of the Western Balkans has been established and is inviolable.” Backing up Ms. Merkel, dozens of prominent scholars and policymakers on both sides of the Atlantic have signed an open letter condemning the proposal and imploring the United States and the European Union to oppose “a return to ethnification of polities and frontiers.”

But there are signs that some Western officials are warming up to the idea. John Bolton, President Trump’s national security adviser, hinted as much last month: “Our policy, the U.S. policy, is that if the two parties can work it out between themselves and reach agreement, we don’t exclude territorial adjustments,” he said. Mr. Bolton is thinking clearly, at least on this front. As long as both the Serbian and Kosovar governments agree to the deal — and can secure sufficient political backing among their publics and legislatures — the United States and the European Union should support it.

Of Kosovo’s population of almost two million, roughly 90 percent are ethnic Albanian and some 6 percent are estimated to be ethnic Serb. Reliable numbers are hard to come by, but around half of Kosovo’s Serbs — high estimates reach 70,000 — live in northern Kosovo, where they make up some 90 percent of the population. Because of its Serb majority, northern Kosovo (about 10 percent of the country’s territory) has been part of the country in name only since independence. Serbia has continued to hold political and economic sway there, leaving Kosovars with a sizable chunk of their country that has no interest in belonging to an independent Kosovo.

Serbia’s Presevo Valley is reportedly home to some 60,000 ethnic Albanians and is comparable in size to northern Kosovo. How much of this area Serbia might transfer to Kosovo is unclear. Nonetheless, trading northern Kosovo for at least some portion of the Presevo Valley would broadly preserve Serbia’s and Kosovo’s current territorial size and population.