NOT surprisingly, Darfur has grabbed the world's attention where Sudan is concerned. The horrors of the past three years, with at least 200,000 Darfuris dead, are unspeakable. But in the longer run another part of Africa's largest country has suffered even more death and misery: Sudan's southern region. Unless strong diplomacy is fast brought to bear there, an even bloodier cycle of despair and destruction could start all over again—and make it much harder to make a lasting peace everywhere in the country, including in Darfur.

Mercifully, an edgy peace has prevailed in the south since 2005, when a “comprehensive peace agreement”, known as the CPA, ended a war between Sudan's Muslim and mainly Arab north and its black Christian and animist south that had stretched back, with the occasional truce, to independence from Britain half a century ago—at a cost of some 2m lives. But that agreement is now hanging by a thread. A complete breakdown could make the debacle in Darfur, in Sudan's west, look modest by comparison. Saving the southern peace accord must be the priority, even while the UN and the African Union prepare for a Darfur peace conference in Libya this month.

A week ago the southern peace arrangement ceased to function when the Sudan People's Liberation Movement (SPLM), which led the southerners in their long war against the northern-dominated Sudanese army, walked out of Sudan's government of national unity in Khartoum, the capital (see article). The peace agreement stipulates that the SPLM and the ruling Islamist National Congress Party, led by the country's president, Omar al-Bashir, must implement the terms of the accord together. It provides for the revenue from south Sudan's abundant oil to be shared equally between north and south, and for a referendum to be held in 2011 whereby the south, if its people so wish, can secede and become a fully independent country. Under the deal, the north promised to remove its troops from the south and to accept independent arbitration to mark the boundary between north and south.

Bashir unabashed

The southerners say, with justice, that Mr Bashir has been trying to wriggle out of the obligations laid down in the accord, just as he has spent years breaking promises to end the conflict in Darfur. In particular, he has refused to abide by a “final and binding” ruling of an independent commission on a new boundary for Abyei state, which straddles the north-south division and holds a vast dollop of Sudan's oil.

The only way to save the north-south agreement is to apply the same relentless pressure on the government in Khartoum that obliged it to sign the deal in the first place and which has now forced Mr Bashir to accept a UN force that is expected to arrive soon in Darfur. The Americans have pushed hardest but, preoccupied by Darfur and crises farther afield, they have relaxed their guard in Sudan's south. They must urgently rally the many governments, Western and African, that have wet-nursed Sudan's north-south agreement and persuade Mr Bashir to meet his obligations over the boundary and the troop withdrawals. They should threaten to apply the same sanctions against Sudan over the south as over Darfur.

Nobody matters more than China. The biggest consumer of Sudan's oil has belatedly started to play its part over Darfur. It is in China's interest to have a stable Sudan. In the short run, some in Beijing may gamble it would be best for their friend Mr Bashir to keep as much of Abyei and its oil as he can. That would be a huge mistake: it would risk a return to a much bigger war than the one in Darfur—and would shut down much of Sudan's oil production, not just in Abyei. If China wants to guarantee its long-term investment, it needs to put pressure on Mr Bashir to do the right thing in all parts of his country.