Trevor Snapp / AFP / Getty Images New recruits for the Sudan People's Liberation Army attend a training session at a secret camp in the Nuba Mountains of South Kordofan on July 11, 2011, in preparation for war against Khartoum

In the shade of a thorn tree on a plain of cracked earth and yellow grass, Brigadier General Namiri Murrad lays out how the rebels of southern Sudan plan to unite and overthrow President Omar Hassan al-Bashir and his Islamist regime in Khartoum. “Right now, our work is to clean our house,” he tells TIME in the embattled region of South Kordofan on April 6, flanked by four captured tanks and pickups that are mounted with heavy machine guns and missile launchers. “The Darfuris are going to clean their house, and the rebels of Blue Nile will clean their house. Then we will move together on Khartoum, and we will finish them. I cannot say when. But I can tell you it’s easier this time — Khartoum is running. They realize they are fighting for the wrong reason. They do not have heart. We are fighting with our hearts. It will be easy to finish them.”

There are reasons to share Namiri’s optimism. Slipping into territory held by Nuba insurgents in South Kordofan, a region of Sudan that borders the newly independent nation of South Sudan, it becomes apparent that a major rebel advance is under way. In the past two months, Nuba fighters from the Sudan People’s Liberation Army–North (SPLA–N) have notched a string of strategic victories, capturing the border town of Jau, the former northern administrative center of Trogi, and pushing back government troops in pitched battles involving thousands of fighters at Korongo, Tess and El Dar. Rebel commanders talk of killing hundreds, even thousands, of Sudanese troops, leaving the plains strewn with bodies — a boast given credence by the number of graves of government soldiers that now mark the sites of recent battles.

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Crucially, the fleeing northern soldiers have left behind an armory of weapons: several tons of shells, mortars and mines; thousands of AK-47s and millions of rounds; artillery and anti-aircraft guns; and 127 pickup trucks in Jau alone, plus four tanks. Major General Izzat Kuku, Namiri’s boss and the acting commander of all Nuba forces, estimates that his soldiers control 80% of the Nuba Mountains, the tribe’s ancestral homeland. In effect, just the two largest cities remain in government hands — Talodi and Kalugli — and an attack on Talodi appears imminent. Namiri says that up to 1,800 troops from Khartoum’s Sudan Armed Forces (SAF) are surrounded by 3,000 of his fighters, plus two more separate forces of between 2,000 and 3,000 SPLA troops. “This is the time of our work,” says Namiri. “This is fighting, and it can always go either way, but I don’t think it will take one week to finish it.”

The implications of the Nuba rebel push are big. Last July, South Sudan split from the regime in Khartoum after more than half a century of war, in which more than 2 million people died. But the new border between North and South left three rebel provinces — Darfur, Blue Nile and South Kordofan, in which the Nuba Mountains are located — in the North. As South Sudan’s independence approached, the northern regime, apparently fearing further loss of power in a concurrent election, launched an offensive on rebels in the Nuba Mountains, who were then observing a cease-fire and even cooperating with the government in joint military units. Khartoum’s security forces first tried to disarm Nuban fighters, then went house to house allegedly arresting and killing Nuban political leaders and activists before ordering an all-out assault on the rebel territory.

TIME sneaked into South Kordofan last June and gathered testimony from more than 30 people on how northern troops were pounding Nuban villages and columns of fleeing refugees with Russian-made Antonov bombers and fighter planes, plus attack helicopters and artillery. Returning this month, TIME obtained an audio recording of a speech by South Kordofan Governor Ahmed Haroun, broadcast on government radio in October and November, in which he exhorts his troops, “I salute the Antonov, the gunships, the MiGs and artillery, supporting you by bombing villages. When you go on your mission, if you find them, kill them, sweep them away, eat them. Do not bring me any prisoners of war. We have no quarter for them.” When TIME relayed a summary of its reporting in June to Luis Moreno-Ocampo, chief prosecutor at the International Criminal Court (ICC), he noted that the court had already issued arrest warrants for al-Bashir and Haroun for allegations of similar conduct in Darfur. Jehanne Henry, Sudan specialist at Human Rights Watch, adds, “It certainly appears war crimes are being committed. The government is not discriminating at all between military and civilian. It seems to have decided to take them all out.”

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If the Nuba rebels are pushing Khartoum’s forces out of South Kordofan, that should allow for an influx of humanitarian relief, which will be desperately needed in the coming months, since Khartoum’s bombings have prevented many villagers from farming. But if the insurgents’ advance in South Kordofan leads to a united rebel offensive against Khartoum and an attempt to bring down al-Bashir’s regime, that will have global significance. Sudan’s rebels would then be attempting to accomplish what the ICC, decades of sanctions, high-volume celebrity advocacy, endless human-rights investigations and even, in 1998, a U.S. bombing of Khartoum have failed to do: reform, tame or even topple one of the world’s pariah states. Chief among its crimes: hosting Osama bin Laden for five years in the 1990s, enriching the center of the country at the expense of its peripheries and trying to impose by force an Arab-dominated, Islamist uniformity on a population whose patchwork of faiths, languages and ethnicities were the definition of heterogeneity.

That, Namiri and Izzat say, is the rebel plan. On Nov. 12, 2011, the Nuba SPLA–N formed an alliance with rebels from Sudan’s two other southern states — including SPLA–N fighters in Blue Nile, to the east, and fighters from the Justice and Equality Movement (JEM) in Darfur, to the west. They called the united force the Sudan Revolutionary Front (SRF). One of its leaders, Yasir Arman, declared that “all Sudan is a theater for [its] operations, including Khartoum.” Izzat tells TIME the SRF has contacts with opposition groups in Khartoum that “may organize people in an uprising — though this will not happen unless we get there and they have people to protect them.”

Izzat, Namiri and several other rebel commanders interviewed by TIME insist that support from their former comrades in South Sudan is limited to advice, food and fuel supplies, medical treatment and diplomatic support. This is not, they insist, a South Sudanese proxy war against its old enemy, pointing out that the South is still overtly fighting Khartoum’s troops at several places along its border. Moreover, the sheer amount of weaponry the Nuba rebels have captured from Khartoum’s troops — seen by TIME — would seem to suggest that, at least for now, they have no need of more lethal outside support.

(MORE: With Time Running Out, South Sudanese Are Stuck on the Wrong Side of the Border)

Khartoum is a long way away from the Nuba Mountains, and rebels, like the politicians they hope to become, have a habit of overstating their chances of victory. Though several hundred JEM fighters helped the Nuba rebels take Jau, it’s far from clear that the dynamics of the new alliance would survive the test of war. Perhaps most important, fighting the Sudanese army in its own territory is a very different proposition than fighting it in areas it has never truly ruled and where it lacks popular support. Nevertheless, after decades of fighting, the rebels are confident the momentum is with them. “They made the wrong calculation when they attacked us,” says Izzat. “A northern conscript soldier who has done two weeks or a month of training cannot defeat a guerrilla force that’s been fighting for 27 years. One soldier of ours can defeat a platoon of theirs. We will all go together to Khartoum. If not this year, then next year or the year after.”

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