THE PEACEFUL birth of a new state in southern Sudan — which has been a diplomatic success story for the Obama administration and the United Nations — is suddenly in danger. On Sunday, forces of the northern Sudanese regime of Omar Hassan al-Bashir invaded the disputed district of Abyei, which is claimed by both the north and the soon-to-be-born state of South Sudan. The town was sacked and burned, and most of its population of more than 15,000 was reported by the U.N. to have fled southward.

Just such a conflict over the fertile and oil-producing territory has long been regarded as the most likely way for Sudan’s partition to turn violent. The question now is whether the Obama administration, which has led international diplomacy on South Sudan, can bring enough pressure to bear on Mr. Bashir to end the conflict before it worsens.

The Sudanese dictator, who is under indictment for crimes against humanity by the International Criminal Court, did not act without provocation. Last week a convoy of U.N. and northern Sudanese forces leaving Abyei was ambushed by southern forces; the Bashir regime claims 70 of its soldiers were killed. But Mr. Bashir seized on that incident to carry out a far more provocative action, the seizure of a piece of ground that has been one of the most contentious issues between North and South. Though most of Abyei’s population is, like most of South Sudan, ethnically African, nomadic Arab tribesmen loyal to the North regularly pass through the area. A referendum on the future of the territory was scheduled for January, on the same day the south voted for independence, but it could not be held because of disputes over who would vote.

To its credit, the Obama administration has responded swiftly to the crisis, helping to forge a Security Council statement Sunday that demanded “the immediate withdrawal of all military elements from Abyei,” while blaming both sides for the violence. On Monday the administration’s special envoy to Sudan, Princeton Lyman, told reporters at a State Department briefing that steps Washington has promised Mr. Bashir in exchange for cooperation with South Sudan, including help with debt relief and the naming of an ambassador to Khartoum, won’t go forward “if we don’t have Abyei being negotiated rather than occupied.”

That is the right message. But the administration must also move to restrain the southern Sudanese government from responding militarily, and urge Arab states and China — north Sudan’s prime economic partner — to use their leverage on Mr. Bashir. No one, with the possible exception of Sudan’s strongman, has an interest in the disruption of South Sudan’s move toward independence, much less another war in Africa.