Goongbaar, Sudan - Hidden behind a cluster of mud huts on the banks of Sudan's White Nile, youthful soldiers perch atop Soviet-era battle tanks and anti-aircraft-mounted trucks, the dark contours of their faces a wavy blur under the midday sun.
They're members of Southern Sudan's military, a rebel army that waged 20 years of guerrilla warfare against Sudan's northern government. But today, these young men are neither fighting a rebellion nor have they laid down their arms.
They're waiting in suspense, as they have for most of the last six years, after a U.S.-backed peace deal created a cease-fire arrangement that's set to end in about three weeks.
"We have an objective beyond this, and we want to achieve it at all cost," said Maj. Gen. Gabriel Jok Riak, a boyish-faced general who commands the first of 10 divisions in the Southern Sudan army, known still by its rebel moniker Sudan People's Liberation Army.
The general is referring to the peace deal's grand finale: a referendum. On Jan. 9, Southern Sudan is set to vote on seceding from the rest of Sudan and forming a new country. According to the deal, independence then could come six months later.
Until then, a wary standby has escalated into a fidgety anticipation, the uncertain wait for a dreaded new round of violence from a volatile opponent.
Riak's concern is northward. Just eight miles north of the riverside army base, a stack of tires in the middle of the road marks the de facto point where Sudan's southern region ends and the northern one begins. Amassed on the other side are these soldiers' old wartime foes, the Sudan Armed Forces. It's the stalled conflict's northernmost front line.
In July, it could become the world's newest sovereign border. The Sudan People's Liberation Army is hoping to secure the split, but some fear that it won't be so easy.
"Sooner or later they will try to disrupt it (the secession process), either during or after the referendum," the general said. "Don't be surprised."
The desire for freedom from northern rule is widely shared among most Southern Sudanese, not just among its military. Resentment of the north is rooted in a painful past: Two million are thought to have died during two long civil wars between the sides when southerners rebelled against the colonially imposed Arab government on the eve of Sudan's independence in 1956.
Most are now eager to put the murderous coexistence to rest.
"We want separation," said Salah Abdel Salaam, a boat captain who lives in Renk, the nearest town, where the Sudan People's Liberation Army division headquarters is based. "All other systems have been suffering and death."
Analysts point out that both sides would benefit from an amicable divorce, including the sharing of oil revenue from the south. Some signs are positive: Sudanese President Omar al Bashir has promised to recognize the referendum result, and the polling process itself has moved forward, with voter registration ending earlier this month.
But other signs seem to point in the opposite direction.
According to the research of the Geneva monitoring group Small Arms Survey, the north is building up its forces along parts of the border and has intensified its practice of arming loyal Arab tribes nearby.
"No one really knows how the north will respond to a likely vote for southern secession," said Claire McEvoy, the director of Small Arms Survey's Sudan project. "That is the big question."
U.S. officials are hoping to promote stability by offering Bashir incentives to let the process proceed peacefully, such as removing Sudan from Washington's list of state sponsors of terrorism.
At this border point at the peak of Southern Sudan's Upper Nile state, both sides face off far from the closed doors of international diplomacy, and anything seems possible.
On Oct. 30, the area was the site of a brief skirmish between small contingents of the two armies. The following day, the two sides met in the middle and agreed to withdraw their front lines to a few miles from the border to quell the situation quickly.
The military positions here are highly strategic for both sides, capping off a chimney-like jut of Southern Sudan land that follows the White Nile River deep into northern territory. The north surrounds the Sudan People's Liberation Army division on three sides.
The capital, Khartoum, is due north, 230 miles straight up a paved highway. Due south, behind Sudan People's Liberation Army lines, sit Southern Sudan's most productive oil fields, which account for more than half of Sudan's output. Under the peace deal, neither side's military is allowed to patrol within the south's oil fields, leaving them effectively unguarded.
In a frank interview at his home, Riak gave a blunt assessment of his troops' ability to defend his immediate position in case of a full-frontal attack.
"How? Who controls the airspace?" he said. "We know the fight would be a disaster for us at first."
But the middle-aged man, who joined the movement in 1983 when he was barely into adulthood and sports scars as if they were medals of honor, wasn't admitting defeat. If the war is brought back to the south, the Sudan People's Liberation Army will join with other rebel movements in Sudan's Darfur region and other marginalized areas, he said.
"If war resumes, we will try to punch a hole and go north," the general said. "It would be a progressive suicide."
The threat could be more than bluster. Recently, the Sudanese government has accused Southern Sudan's leadership of actively aiding the largest Darfur rebel group, the Justice and Equality Movement, a charge that Southern Sudan denies. The north's ruling party also has complained that many of the Darfur rebels have taken refuge in Juba, Southern Sudan's capital.
One of these new Juba residents is Minni Minnawi, a splinter leader of the Darfur rebel group the Sudan Liberation Army who signed a 2006 accord with the government, the Darfur Peace Agreement. This week, he announced that the peace deal was dead and that he was in talks with the Sudan Liberation Army's founder, Abdel Wahid al Nur, to recombine the rebel group.
The Sudan People's Liberation Army also maintains close political ties with rebel comrades who joined the rebellion but now are stuck north of the division line.
One, former Sudan People's Liberation Army Gen. Malik Agar, is the governor of the neighboring Blue Nile state.
If the Sudan People's Liberation Army is strengthening ties with armed disillusioned groups in the north, it could be a twist in a familiar playbook. During the war, Bashir heavily armed breakaway rebel factions to serve as counterinsurgency proxies.
"The south could be saying, 'Look, you've got tools, but we've got tools too,' " said Fouad Hikmat, a Sudan analyst for the International Crisis Group, a nonprofit crisis-resolution group.
The message is clear: Don't try to stop the secession, or else.
"Khartoum would fall," Riak said. "And then we would be part of the new ruling coalition," he said, with a slight smile.
For now, his troops continue their years-long vigil, nearing what people here call their "final walk to freedom."
David Aolach Bion, a 34-year old low-ranking soldier, stood guard nearby.
He spent 11 years as a refugee getting educated in Uganda. That makes him rare in a force that's overwhelmingly illiterate. He's also an aspiring poet.
"God is a single-breast cow," read one of his compositions that he shared eagerly on a crumpled piece of paper. "That is why there's the gap between the rich and the poor."
When he was asked why he returned to the military, David Aolach's answer needed no literary interpretation. "I came back to demarcate the border," he said.
(Boswell is a special correspondent. His reporting for McClatchy from Sudan is partially funded by a grant from the Humanity United foundation, a human rights group based in California.)