After the 2005 Comprehensive Peace Agreement that ended the North-South war, Salva Kiir, the current president of South Sudan, built a country out of a fragile coalition of tribes and regions. I served as the United States envoy to Sudan from 2006 to 2007, and the Salva Kiir I knew then was committed to democracy, human rights and the rule of law. But now his critics say he has concentrated power in his own hands, using repression instead of persuasion to rule.

Mr. Kiir comes from South Sudan’s largest tribe, the Dinka, while his former vice president, Riek Machar, comes from the second-largest tribe, the Nuer. These two groups have a history of economic and political rivalry, and of bloody confrontation. In 1991, after a power struggle among the southern rebels, Mr. Machar broke away from John Garang and Mr. Kiir, the leader and his deputy, respectively, of the Sudan People’s Liberation Army (S.P.L.A.) and Sudan People’s Liberation Movement (S.P.L.M.), and allied his Nuer militia with the North.

It was a marriage of convenience: Mr. Machar supported Southern independence, while Mr. Garang and Mr. Kiir supported autonomy for the South within a united Sudan. Khartoum opposed independence, but it armed Mr. Machar to keep South Sudan in chaos. The southern forces fought among themselves for a decade, committing terrible atrocities: In the 1991 Bor massacre Mr. Machar’s Nuer forces murdered thousands of Dinka civilians.

The southern forces finally united in 2001, under pressure from the U.S. government. After the peace agreement with Khartoum was signed in January 2005, Mr. Garang became the president of the semi-autonomous, interim government of South Sudan. When he died that July in a helicopter accident, Mr. Kiir quickly succeeded Mr. Garang, and Mr. Machar became his vice president. They worked together to balance sectarian interests within the South, particularly in the distribution of jobs and government contracts. They also worked together to defuse tensions when communal violence threatened stability.

The South became fully independent on July 9, 2011. But the public’s expectations of a quick peace dividend were disappointed. In 2012, Mr. Kiir accused some of his ministers and army generals of stealing $4 billion. The government has been using repression to silence its critics. The S.P.L.A. has committed human rights abuses in Jonglei State, in eastern South Sudan, against the Murle, a tribe armed by Khartoum to cause chaos in the region.

In March, Mr. Machar criticized Mr. Kiir’s authoritarian leadership; he also announced that he would challenge Mr. Kiir for the S.P.L.M.’s chairmanship and run for president in 2015. Mr. Kiir fired Mr. Machar as vice president in July, accelerating the collapse of the fragile government’s tribal balance of power.

Mr. Kiir’s critics in the South argue that he has driven out the reformers in his party and is surrounding himself instead with loyalists from his home area and former acolytes of President Omar al-Bashir of Sudan. Mr. Kiir has made some compromises with Khartoum over sharing oil revenues and the status of the contested border region of Abyei. He might argue this was to stabilize South Sudan’s relationship with Sudan, but others say he has conceded too much.

Although the quarrel between Mr. Kiir and Mr. Machar has no ethnic basis, their political fight is now mutating into a tribal war. Tensions between the Dinka and the Nuer within the presidential guard blew up on Dec. 15, after Mr. Kiir ordered Nuer soldiers to disarm because he questioned their loyalty. They did, but then Dinka soldiers picked up the weapons and some shooting occurred.

Mr. Machar and his soldiers have since escaped to the Nuer homeland in the Upper Nile and Unity states. The United Nations and the media report that Nuer militias aligned with Mr. Machar have committed widespread retaliatory atrocities against Dinka civilians. Mr. Machar’s troops have also taken control of the oil fields in the Nuer homeland, a formidable tool of leverage since oil accounts for more than 90 percent of Juba’s revenues.

President Bashir of Sudan is said to have proposed to Mr. Kiir sending troops from the North to protect the Southern oil fields. Mr. Kiir refused, wisely, but now Mr. Machar has announced that he intends to negotiate his own oil deal with Khartoum. This will allow Khartoum to play the southern sides against one another.

The United Nations Security Council decided on Tuesday to send more troops to South Sudan to ensure the protection of civilians. But much more needs to be done.

Mr. Kiir must release all political prisoners from the S.P.L.M. He also should put in place an interim government until elections can be held. Mr. Machar, for his part, must cease all offensive military operations and withdraw his troops from the oil fields. If he refuses, the United Nations should impose sanctions.

Although Mr. Kiir and Mr. Machar say they support reconciliation, they are both trying to make military gains first in order to shore up their negotiating positions. Thus talks must be organized as soon as possible. President Yoweri Museveni of Uganda has offered to act as mediator, but Ugandan media report that he has sent his own special forces to protect Juba from Mr. Machar’s approaching forces. He is not neutral, and another broker should be found — perhaps the Ethiopian government.

A decade ago, Mr. Bashir argued that the South should not be granted independence because the Southerners could not govern themselves and would lapse into ethnic conflict. They must not prove him right.