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The road to Heglig has no sign or post marking the border between northern and southern Sudan, where Sudan's new war began on Saturday. Instead, there is a sudden trail of rotting corpses leading steadily north. At its head stands a northern Sudanese military base, now captured and looted by the South. Inside, South Sudan's generals plan their next offensive, marking troop positions and movements in the sand with a curtain rod. Outside, South South Sudanese soldiers mix freely with their allies — officially denied, but now in open view — from the Darfuri rebel group, the Justice and Equality Movement (JEM). The men are wary. They glance at the sky to check for approaching northern warplanes, and dig shallow foxholes for protection against bombs. Suddenly a Sudanese jet screams overhead. The dry desert air erupts with the thud of an aerial bombardment. "We are under attack," yells Maj. Gen. Mangar Buong, the South Sudanese commander. And the soldiers scatter for cover.

Sudan, once again, is back at war. Whether the conflict lasts for days, weeks or years is unknowable. What is clear is that the pretense of peace can no longer be maintained. Sudan's northern regime in Khartoum fought the South for more than half a century in a conflict that cost 2 million lives. The pair have been officially at peace since a 2005 agreement that led, last year, to the separation of north and south into two separate countries. But the border between the two remains disputed at several places and the two sides have fought sporadic skirmishes along their frontier for years.

In recent months, the most deadly of these have centered in and around Heglig, an oil field officially in the north but claimed by the south. Northern and southern soldiers have exchanged fire, northern bombers have attacked southern territory — and last Saturday southern soldiers invaded and seized Heglig. South Sudan claims it is just defending against northern aggression. But that claim is weakened by the presence of JEM, whose agenda is nothing less than the overthrow of President Omar al-Bashir's regime in Khartoum. Like turbaned cowboys, JEM's fighters swarm up and down the road in roofless pick-up trucks mounted with heavy guns. Arabic is sprawled across their trucks, and they point and pose for photos, exclaiming "Darfur" or "JEM." Officially, South Sudan says it has no ties with the Darfur rebels. In Heglig, that's another pretence that no longer stands up.

The big question: is Bashir's regime really in danger? Maj. Gen. Mac Bol, South Sudan's deputy military intelligence chief, says the South plans to push up to 90km north of Heglig. Across South Sudan, a full-scale southern mobilization is underway. Hundreds of soldiers stream up towards the Heglig front lines every day in giant trucks. According to multiple sources, both sides are also sending heavy reinforcements to other border hotspots, particularly Raja in the west and Renk in the east. Carol Berger, a post-doctoral researcher at Bristol University in Britain, reports seeing convoys of soldiers traveling through central South Sudan. "They're taking any transport they can find, even the local taxis, to get back to their barracks," Berger said. One source close to South Sudanese President Salva Kiir warned TIME: "This isn't going to be over anytime soon."

The offensive only compounds the woes of Khartoum's overstretched military. Already this year government forces have been routed in a series of pitched battles on another front to the northeast, in the Nuba Mountains in Southern Kordofan. There the Nuba rebels are open about their intent to flush Khartoum's soldiers out of their territory and march on Khartoum — and open too about their alliance with JEM and other rebels to the east, in Blue Nile state. (The Nuba admit South Sudan does lend them diplomatic, medical and advisory support while denying any military backing — though they admit that most of their weapons come from their days in the South Sudan military, before the current rebellion.)

Even if a march on Khartoum fails to materialize from either of these fronts, the offensive puts pressure on Khartoum in another way: shutting down the Heglig oil fields and starving the Sudanese government of revenue. South Sudan already shut down its oil fields earlier this year, denying itself revenues of $4 billion a year but hitting Khartoum in the pocket too, since the South's oil is pumped out through the north, a service for which Khartoum extracted billions of dollars in fees. The screws could be tightened still further. Most of Khartoum's other oil fields also lie near the border with South Sudan, and could also become targets.

For the world powers who invested years and billions of dollars in peace in Sudan, the situation has rarely looked bleaker. The U.S. in particular sponsored the 2005 peace deal. Seven years later, diplomats are wondering whether all they achieved was an extended breathing space to allow both sides to re-arm and re-finance before returning to the fighting. US special envoy to the two Sudans, Princeton Lyman, looking tired, admits this is war. "[And] one of the dangers is that it is a war that will spread well beyond Heglig [and] get nastier and nastier."

Khartoum, as usual, is on the receiving end of U.S. criticism. Lyman, who was to visit the Sudanese capital later Wednesday, describes the attitude there as "very belligerent." But if anything it is South Sudan — created out of the peace that the U.S. brokered only to use its new freedom to go back to war — with which the U.S. is most frustrated. For years, U.S. policy on Sudan had a heavy southern bias — with good reason. Khartoum, once home to Osama bin Laden, has an atrocious record of war crimes, atrocities, corruption, all fueled by a racist, Arab-supremacist outlook. In that context, South Sudan was a victim to be protected. Since the end of hostilities in 2005, however, a very different creature has emerged in the south: a regime every bit as corrupt as the north, with a predatory army and the same flair for ruthless political brinkmanship, which — dominated by an ethnic Dinka majority — is either unwilling or unable to stop a spate of tribal violence that has left hundreds, if not thousands, dead so far just this year. "Our good friend South Sudan has become extraordinarily impatient," says Lyman. "We think they've taken great risks." Lyman, the U.N. and the international community are united in their desire for peace in Sudan — as are the majority of Sudanese. So far, there is no sign that the leaders on either side are listening.

• Alan Boswell / Heglig
• Over the weekend, the conflict broke into the open. Now Bashir's enemies say they want to march right up and take Khartoum