TWO YEARS into its existence as an independent state, South Sudan is teetering on the edge of the abyss. Often described by U.S. officials as a hopeful example of a new democracy forged out of decades of civil war that cost millions of lives, South Sudan and its adversary, Sudan, are again mired in human suffering and conflict.

The latest crisis erupted this week when South Sudan’s president, Salva Kiir, a veteran rebel commander, fired the cabinet, the vice president and the top negotiator with Sudan. The move was apparently the climax of a power struggle with Vice President Riek Machar, a rival rebel commander who had been critical of Mr. Kiir and is a potential challenger in the 2015 election. Mr. Kiir had stripped the vice president of some of his duties in April, but the dismissal came abruptly and may foreshadow more instability.

Most ominous, the export of oil from South Sudan through Sudan is about to cease, which could deal a crippling economic blow to both countries. The oil resumed flowing in April after a long dispute over revenue-sharing. South Sudan gets almost all its government budget from the export of oil, but it relies on the Sudan pipeline, and the flow has been halved in recent weeks. This time, Sudan says it is halting the flow of the south’s crude to a Red Sea port after Aug. 7 because of the south’s support for rebels fighting the government in Khartoum. South Sudan denies backing the rebels and accuses Khartoum of doing the same.

At the same time, a humanitarian disaster has erupted in South Sudan’s Jonglei state, where clashes between government soldiers and insurgents, as well as tribal violence, has forced tens of thousands of people to seek refuge in areas where rains have washed out roads and airstrips. Hiding in the bush and swamps for weeks, they are desperately in need of food. The U.N. World Food Programmeannounced it is attempting to rush food into these areas by helicopter but is seeking $20 million in emergency aid for the aircraft and supplies.

In a statement Wednesday, the State Department said it was “deeply concerned” and urged South Sudan to “stay true to the vision it laid out for itself two years ago,” with “sustained commitment to democracy and good governance, justice and accountability, and respect for rule of law and the human rights of all of South Sudan’s people.” Nice words, but the administration has been without a special envoy to Sudan and South Sudansince December; a spokesperson said a new appointment will be made shortly.

In his June 30 speech in Cape Town, President Obama urged Sudan and South Sudan to get on with a lasting peace, “not just words on a paper or promises that fade away.” More than presidential rhetoric will be needed to make that happen.