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.