Bosnia, in Peril Once More Early in President Obama's first term, in May 2009, Vice President Joseph R. Biden Jr. paid a visit to Sarajevo, the capital of Bosnia and Herzegovina. Addressing Parliament, he told Bosnian, Serbian and Croatian members, "My country is worried. ... We have seen a sharp and dangerous rise in nationalist rhetoric. Such inflammatory speech, he said, "must stop." Otherwise, he warned, "You will descend into the ethnic chaos that defined your country for a decade." Since then, other than the inauguration of a new American Embassy in Sarajevo by Secretary of State Hillary Rodham Clinton in 2010, there has been little engagement by the United States and no follow-up to Mr. Biden's words. Instead, the situation Mr. Biden described is being enacted: Bosnia is slipping back into a dangerous sectarianism. Milorad Dodik, president of Republika Srpska, the Bosnian Serb entity, is rewriting the narrative of the war - claiming that Muslims started it. Mr. Dodik denies Serbian responsibility for genocide in the 1995 Srebrenica massacre of 8,000 Muslim men and boys; he recently testified on behalf of the former Bosnian Serb leader Radovan Karadzic, on trial in The Hague for war crimes. Eighteen years ago, it was American diplomacy, backed by force, that ended Europe's most savage conflict since World War II. The 1995 peace treaty forged in Dayton, Ohio, stopped ethnic cleansing - the term coined in those Balkan wars - and gave birth to a new country. Now, while Bosnia and Herzegovina's fragile unity is fraying, the international community is as disengaged as when war first erupted in 1992. The United States has much at stake; it needs to return to Sarajevo. Superficially, peace and prosperity reign in Sarajevo. Its skyline of mingled minarets, Orthodox domes and synagogues belies the West's convenient myth of Bosnia's "ancient ethnic animosities." The city has been largely rebuilt, with a fine opera house and one of the world's richest libraries. But a visit to the Sarajevo tunnel, built by the Bosnian Army to resupply the besieged city surrounded by Serbian forces, is a ready reminder of how quickly that shiny metropolis once turned into a city of cave dwellers. The tunnel is a dark and damp hole in the ground through which 4,000 Sarajevans hauled potatoes and guns each day, crawling in mud for 40 minutes each way, to keep from starving. The European Union has now claimed authority over the country, and the United States has yielded. This is akin to returning a child to abusive parents. Sarajevans still recall Sir David Owen, one of a string of European "peace" negotiators, warning them not to dream of Europe's coming to their rescue. So, for nearly four years, Serbian snipers were able to pick off women and children as they lined up for bread. The siege lasted three times longer than Stalingrad and cost 11,000 lives before Washington engaged and ended the massacre. During those negotiations, the architect of the Dayton Accords (and my husband), Richard C. Holbrooke, sometimes asked me to take whichever of the warlords he was having trouble with for a walk: "Make him talk about his dreams for his children and grandchildren." That proved impossible: These men were entirely focused on war, on holding on to land and power. The current crop of Bosnian politicians reminds me of those I knew in Dayton. True, the Dayton Accords have held the peace, but no treaty can serve as a nation's permanent constitution. Richard predicted it would be only as good as its implementation - and Dayton is being neither implemented nor updated. Washington's current policy, as declared by Assistant Secretary of State Philip H. Gordon in Sarajevo in 2011, was summed up by an embassy official for me as "Pull your socks up!" Bosnia's fractious Parliament can't agree on words to a national anthem, much less common foreign or economic policy. Meanwhile, Washington's disengagement has only emboldened nationalists like Mr. Dodik. "They are just playing with our wounds," Orhan Pasalic, a Bosnian soldier turned banker, told me. Muslim, Serbian and Croatian leaders address only their own peoples. No one speaks to the country as a whole. In Sarajevo, Mr. Pasalic's children no longer share classrooms with other ethnicities. They have no personal memories of war's horror, yet they will soon face an economy with 40 percent unemployment and may not stay passive for long. How long before the next Milosevic lights the fuse? The European Union is empowered to stop this slide. Yet its high representative, Valentin Inzko, who has broad powers under Dayton, spends roughly two days a week in Sarajevo. For the rest, I hear, he tends his stables outside Vienna. If Bosnia disintegrates - already the Republika Srpska threatens to secede - the guns may return to the hills around Sarajevo. Then another flood of refugees will pour across European borders, and Europe will again be mired in an avoidable, predictable spasm in the Balkans. After more than a decade of the "war on terror," the United States is wary of intervention, but no one is advocating American boots on the ground. In Bosnia, where we did once intervene to stop the massacre of Muslims, all it will take is the kind of determined diplomacy that ended the war in 1995. Only the United States, which has so much moral authority invested in this fragile peace, is up to that job. Kati Marton is a former chairwoman of the Committee to Protect Journalists and is the author, most recently, of "Paris: A Love Story."