U.S. officials in Juba, while acknowledging many of the government’s shortcomings, defend South Sudan. They point to its painful history of war and say it’s constantly being provoked by the Sudanese government, led by President Omar Bashir, himself wanted by the International Criminal Court for alleged war crimes. They point out that Sudan backs dissident South Sudanese militias, has bombed South Sudanese territory, has invaded the disputed territory of Abyei and has blocked all trade with landlocked South Sudan.

"We knew the newest country in the world would face many challenges, given that South Sudan ranks at the bottom of almost all development indicators worldwide. But our commitment to help the South Sudanese people realize their aspirations remains strong," said Princeton Lyman, the U.S. special envoy to Sudan and South Sudan, who’s based in Washington.

Former Bush administration officials also remain firmly in South Sudan’s corner. Two – Andrew Natsios, President George W. Bush’s special envoy to Sudan, and Jendayi Frazer, Bush’s assistant secretary of state for Africa – have called for the Obama administration to provide South Sudan with anti-aircraft weapons to ward off Sudanese warplanes. South Sudan has no air force.

Frazer and Natsios back that stance by arguing that Sudan, not South Sudan, was most to blame for the April fighting.

"They (Sudan) were using Heglig to attack border areas in the south. They were using it in their bombing campaigns. When you bomb another country, that’s an act of war," Natsios said.

However, mounting evidence that South Sudan’s foray into Sudan had been planned for weeks, if not months, is undermining that argument.

In a series of interviews over the last three months, diplomats and other knowledgeable officials have made clear that the South Sudanese carefully coordinated the capture of Heglig with members of the Sudanese rebel group Justice and Equality Movement, whose cause has long been the liberation of Sudan’s conflicted Darfur region.

Officials on the ground say that Justice and Equality Movement rebels began arriving en masse on the South Sudan side of the border near Heglig in February, basing themselves in the state capital of Bentiu, clearly under agreement with South Sudanese authorities.

During the fighting in April, Justice and Equality Movement forces manned checkpoints in South Sudan side by side with South Sudanese soldiers, and, once, when members of a rebel convoy forcibly stole two cameras from a photojournalist, the South Sudanese authorities were able to return the equipment by the next day.

South Sudan’s attack northward with the rebels "was definitely planned," said one Western security official, who spoke only on the condition of anonymity since he wasn’t authorized to talk on the record.

“They are both the bad guys now," he said, referring to Sudan and South Sudan.

A senior African diplomat characterized the Heglig offensive as “part of the large, grandiose plan to bring the north down." He said South Sudanese officials "aren’t thinking as a nation-state, but as a liberation movement."

The diplomat, who agreed to be interviewed only on the condition that he not be identified because he didn’t want to damage his relationship with South Sudanese officials, said the plan began with South Sudan’s decision to shut down oil production in January rather than agree to an export deal that would heavily compensate Sudan. When the two nations split, South Sudan received most of the oil-producing areas, but it had to ship the oil through Sudan because South Sudan has no ports.

The diplomat said he suspected South Sudanese officials hoped that the cutoff of financial resources, coupled with the Heglig incursion, would be enough to spark a more generalized uprising in the north.

Izzat Kuku, a senior rebel commander in Sudan’s Nuba Mountains, told McClatchy before the Heglig capture that the Justice and Equality Movement was preparing to move north through Heglig and link up with other rebels to march north. Other sources confirmed that plan. The Justice and Equality Movement rebels were in Heglig to begin that mission, they say, and meetings to plan an offensive among senior Nuba, Darfuri and South Sudanese military officials began in January.

Such cavalier behavior wouldn’t be new, according to Alex de Waal, a leading Sudan scholar and an adviser to the African Union mediation team. John Garang, the South Sudan rebel leader who signed the 2005 peace deal and died months later, was pursuing an uprising in Khartoum and an insurgency in Darfur even as he was negotiating the final peace deal with Bashir, de Waal said.

Garang’s followers "similarly keep several horses running at the same time, and their hope is that a configuration of events one day will allow them to gamble on winning the big prize," de Waal said.