At a most inopportune time, the Balkans are back. On December 10, the U.S.-E.U.-Russian negotiating team tasked with getting the Serbs and Albanians to agree on Kosovo’s future status will report to the United Nations that it has failed. A few weeks later Kosovo’s government will proclaim that Kosovo is an independent nation — a long overdue event.

America and most of the European Union, led by Britain, France, and Germany, will recognize Kosovo quickly. Russia and its allies will not. Kosovo’s eight-year run as the biggest-ever U.N. project will end with great tension and a threat of violence that could spread to Bosnia.

Because security in Kosovo is NATO’s responsibility, there is an urgent need to beef up the NATO presence before this diplomatic train wreck. Just the thought of sending additional American troops into the region must horrify the Bush administration. Yet its hesitations and neglect helped create this dilemma — which Russia has exploited.

There is more bad news, virtually unnoticed, from nearby Bosnia. Exactly 12 years after the Dayton peace agreement ended the war in Bosnia, Serb politicians, egged on by Moscow and Belgrade, are threatening that if Kosovo declares its independence from Serbia, then the Serb portion of Bosnia will declare ITS independence. Such unilateral secession, strictly forbidden under Dayton, would endanger the more than 150,000 Muslims who have returned there.

Recent American diplomacy led by Undersecretary of State Nicholas Burns and special envoy Frank Wisner, working closely with E.U. negotiator Wolfgang Ischinger, has largely succeeded in persuading most of our European allies to recognize Kosovo rapidly. But NATO has not yet faced the need to reinforce its presence in Kosovo. Nor has serious transatlantic discussion begun on Bosnia, even though Charles English, the American ambassador in Sarajevo, and Raffi Gregorian, the deputy high representative in Bosnia, have warned of the danger. “Bosnia’s very survival could be determined in the next few months if not the next few weeks,” Mr. Gregorian told Congress this month. Virtually no one paid any attention.

The icing on the cake? Russia has threatened to link the Kosovo issue to the claims of two rebellious areas of far-away Georgia, Abkhazia, and South Ossetia.

These issues had seemed largely resolved in the late 1990s. For such extensive backsliding to occur took a poisonous combination of bad American decisions, European neglect, and Russian aggressiveness.

When Serbian dictator Slobodan Milosevic was ousted in September 2000 and a reformist government took over, the road seemed open to a reasonably rapid resolution of Kosovo’s final status. But the new Bush team hated anything it had inherited from Bill Clinton — even, perhaps especially, his greatest successes — and made no effort to advance policy in Kosovo until 2005 and ignored Bosnia. Defense Secretary Rumsfeld even sought to pull American troops out of the NATO command in Kosovo, which Secretary of State Powell prevented. However, the State Department did not prevent Mr. Rumsfeld from prematurely turning the NATO command in Bosnia over to a weak E.U. Force, a terrible mistake.

By the time meaningful diplomatic efforts started in 2006, the reformist prime minister in Belgrade had been assassinated by ultranationalists. And Vladimir Putin decided to re-enter the Balkans with a dramatic policy shift: No longer would Russia cooperate with Washington and Brussels in the search for a peaceful compromise, as it had in 1995 when Bill Clinton and Boris Yeltsin sat on the hillside at Hyde Park and reached a historic agreement to put Russian troops under NATO command. Today, Mr. Putin seeks to reassert Russia’s role as a regional hegemon. He is not trying to start another Cold War, but he craves international respect, and the Balkans, neglected by a Bush administration retreating from its European security responsibilities, are a tempting target.

Mr. Putin was hardly quiet about this; I watched him bluntly warn German Chancellor Angela Merkel, Defense Secretary Robert Gates, and delegates to the Munich security conference in February that Russia would not agree to any Kosovo settlement that Belgrade opposed. There was a vague feeling in Washington and Brussels that Mr. Putin was bluffing — and no real planning in case Mr. Putin meant it.

Not only did he mean it, Mr. Putin upped the ante by extending his reach into the Serb portion of Bosnia. Using some of his petrodollars, Mr. Putin turned its mildly pro-Western leader, Milorad Dodik, into a nasty nationalist who began threatening secession. The vaunted Atlantic alliance has yet to address this problem at a serious policy level — even though, as Mr. Gregorian warned, it could explode soon after Kosovo declares independence.

The window of opportunity for a soft landing in Kosovo closed in 2004. Still, President Bush should make one last, personal effort with Mr. Putin. His efforts must be backed by temporary additional troop deployments in the region. It is not too late to prevent violence, but it will take American-led action and time is running out.

Mr. Holbrooke was the chief architect of the Dayton peace agreement, which ended the war in Bosnia. He is a former U.S. ambassador to the United Nations.