Helping bring peace to southern Sudan in 2005 was the Bush administration's finest foreign policy achievement. But it is now unraveling, risking a new north-south civil war that could surpass Darfur as a political and humanitarian disaster.

The Darfur advocacy campaigns have familiarized the American public with the suffering and abuse visited on civilians in that region of western Sudan. The people of southern Sudan suffered no less during the years of civil war beginning in 1983. The successive governments in Khartoum had two weapons of choice: freelance militias licensed to raid, burn and plunder, and deliberate famine that starved southern Sudanese to the point where vast tracts of their fertile land are now depopulated.

The stakes were undeniably high. Khartoum didn't want to lose control of the south, which has oil. But many of those who live in southern Sudan — Christians and followers of traditional theistic faiths — believe that their homeland should separate from northern Sudan and end generations of exploitation by Khartoum's Arab-Islamic elites. Over 20 years, up to 2 million southerners have perished.

A concerted diplomatic effort by neighboring African countries, backed by the U.S., Britain and Norway, brought Africa's longest civil war to an end. The Bush administration's commitment to peace was pivotal. In his first months in office, President Bush reversed the previous Clinton administration policy of backing Sudan's armed opposition — especially the southern-based Sudan People's Liberation Movement led by John Garang — in favor of a negotiated accord.

U.S. pressure helped make that peace a reality. More important still was a shared vision of a democratically transformed Sudan with a government of national unity that had a place for all, including President Omar Hassan Ahmed Bashir and the Islamists.

The peace deal was signed in Kenya in January 2005. But, as diplomats said at the time, the deal was just a beginning; implementing the agreement would be 10 times harder. It includes provisions for power- sharing, dividing the national wealth, demarcating the internal north-south boundary, integrating government troops and former rebels into joint military units, holding democratic elections in 2009 and holding a referendum in the south on self-determination in 2011.

But Garang died in a helicopter crash, and Vice President Ali Osman Taha, the leading moderate voice in Khartoum, found himself politically marginalized. With Darfur engulfed in war, progress became harder still.

Recent weeks have brought an accelerating drumbeat of warnings that the peace accord is breaking down. Garang's successor as leader of southern Sudan has spoken of a return to war. Southern leaders complain repeatedly that their counterparts from the north, in the National Congress Party, renege on agreements and make key decisions behind their backs.

On Thursday, Pagan Amum, secretary-general of the SPLM, announced that his party was pulling out of the unity government until key elements of the peace agreement were fully implemented. Meanwhile, both sides are expanding their armies, aiming — for now — to deter the other from initiating a war.

Few Sudanese doubt that a new war would be even more hideous than its predecessor. The south would try to secede; for Bashir, it would be a fight to the death.

Millions of southern civilians live in the north, including in and around the capital, Khartoum. The SPLM has supporters and troops in other parts of the north as well, including the highly combustible Kordofan region. That area, next to Darfur, already is suffering a spillover of that war, and just last week the United Nations warned that violent conflict could erupt there.

Potentially compounding disaster, a secessionist war probably would draw in Egypt on Khartoum's side and other neighbors, such as Uganda, in support of the south, and ignite a conflagration throughout the Nile Valley.

The deepening political crisis also poisons the chances for any peaceful resolution of Darfur's conflict. Why should Darfur's rebels make a deal with a government that seems to be collapsing? If it does collapse, the war in Darfur will enter a new and more deadly phase.

The dream of democratic transformation in Sudan is ailing, but it is far from dead. So far, the north-south cease-fire is holding. But the 2005 peace accord urgently needs the full diplomatic strength of the Bush administration behind it — especially if peace in Sudan is to be any part of the Bush legacy.

Alex de Waal is a program director at the Social Science Research Council and editor of "War in Darfur and the Search for Peace." He wrote this commentary for the Los Angeles Times.