Sudan's two-decade civil war killed 2 million people and forced another 4 million from their homes.

Often depicted as a conflict between the Arab, Muslim north and the black animist or Christian south, the war was fought over oil money, political power and religious issues.

Khartoum's countrywide imposition of Islamic sharia law in 1983 was one of the main factors leading to insurrection in the south by armed groups, including the main rebel organisation, the Sudanese People's Liberation Movement (SPLM). But experts say the war dragged on mainly because of oil.

At its height the conflict spread, both politically and militarily, beyond the geographical confines of the south. The SPLM deployed soldiers to eastern Sudan to support fellow insurgents there, and developed links with Eritrea, which is dealing with an Islamic insurgency it says is supported by Khartoum.

Africa's longest civil war ended in 2005 with the Comprehensive Peace Agreement (CPA). But the road to peace has been rocky with both sides accusing the other of violating the accord.

Under the power-sharing agreement, the ruling party in Khartoum, the National Congress Party, gave up half its ministerial posts to the SPLM but held on to the most powerful ministries - energy, defence, justice and internal security.

Both sides agreed to split billions of dollars of oil revenues. Leaders in southern Sudan promised to use the oil money to build much-needed infrastructure and construct 10,000 km (6,200 miles) of roads within six years.

In July 2005, SPLM leader John Garang died in a helicopter crash just three weeks after he had been inaugurated as Sudan's vice-president, prompting fears the CPA would unravel. But in October, Sudan's new constitution was ratified and a new government was sworn in.

Under the CPA, the SPLM has the right to form a southern regional government. The south also has the right to hold a referendum in 2011 to decide whether it should remain part of Sudan or secede.

The referendum is one of the most controversial aspects of the CPA. Some experts think it may never take place because most of Sudan's oil reserves lie in the south, and the north is unlikely to give up control of so much wealth without a fight.

Any attempt at secession, experts warn, could provoke yet another round of violence.

Other sources of tension include the division of national wealth, power-sharing, demarcation of the north-south border and the integration of government troops and former rebels into joint military units.

Relations reached crisis point in October 2007 when the SPLM walked out of the coalition government complaining that Khartoum had not implemented key aspects of the deal.

One of the issues cited by the SPLM was the failure of government soldiers to leave southern oil fields.

Under the peace deal, the fields are supposed to be partrolled by joint units of northern and southern soldiers.

The SPLM rejoined the government in December 2007 after an agreement that the northern troops would leave.

But in May 2008 clashes in the disputed oil-rich town of Abyei, which is claimed by both Khartoum and the Southern government, displaced tens of thousands and re-ignited fears of a new civil war.

Since then, joint forces have begun operating in the region.

As well as the unresolved issues between the SPLM and the National Congress Party, there are other reasons why conflict could break out.

First, the peace agreement recognised only the SPLM, leaving out smaller armed groups in the south some of which still operate.

The potential for in-fighting among southerners was also not addressed, despite well-known rivalry between two main ethnic groups, the Dinka and Nuer. That rivalry has traditionally been exacerbated by the government, which manipulates tension between Sudan's various ethnic groups for its own advantage.

Conflicts between other tribes also continue throughout the south, and many communities have refused to disarm as a result.

Second, the CPA ignored the grievances of about 30 armed groups around Sudan, whose demands for a share of power and resources are similar to those of the SPLM.

Experts say the government's refusal to include such groups in the peace negotiations was partly to blame for the uprising in Darfur, where rebels were left out of the carve-up of power and resources.

Another complication for SPLM government ministers was the group's support for rebels in eastern Sudan, who threatened to spark a conflict unless their grievances were heard and concessions extracted from Khartoum. The rebels have since signed a peace deal with the government.

Peace in the south has meant more than half of the 600,000 refugees scattered throughout six neighbouring countries have returned home. Another 2 million internally displaced people have also returned.

The first part of 2008 in particular saw a surge in returnees, as people headed home to be counted in a national census in April.

But hundreds of thousands remain displaced, many reluctant to return to a region still devastated by the war.

South Sudan is a bleak place to live, with widespread hunger and staggering mortality rates. Many communities lack clean drinking water, land to farm, work opportunities, health facilities and schools.

The U.N. Population Fund said in 2007 south Sudan had the highest maternal mortality rate in the world - with 2,030 women dying per 100,000 births. That compares with 509 deaths in the north.

According to Larry Thompson of Refugees International, south Sudan ranks near the bottom in the world in all social indicators including education, literacy, and child malnutrition.

The situation has been worsened by Khartoum's hostility towards international humanitarian operations. This has translated into limited resources and facilities to deal with the potentially destabilising return of millions of displaced people.

Another factor adding to insecurity in the south after the war is the presence of the Lord's Resistance Army, a Ugandan rebel group. The cult-like LRA, which waged a 20-year insurgency in northern Uganda, has used bases in Sudan. In mid-2007 many rebels moved to a base in Democratic Republic of Congo, but continue to attack villages in south Sudan, threatening delivery of aid.

The existence of lucrative oilfields in the south has led experts to wonder whether Sudan will be affected by what's known as "the oil curse".

Oil is hardly a one-way ticket to the top of the development league tables. In fact, experts say the discovery of oil can be a recipe for disaster - leading to social and political upheaval, infighting and corruption.

Oil often seems to make the rich richer and the poor poorer. Too often, oil dollars wind up in private bank accounts and hinder rather than enhance development.

It can also fuel conflict, as it has in other countries on the continent including Angola, Nigeria and Chad.

And dependency on oil leaves countries vulnerable to external shocks - anything from oil price falls to natural disasters or conflict that interrupts trade.

That's why Sudan's oil could be a curse rather than a blessing.

The 2005 agreement stipulates that the south should share the profit from oil produced in Southern Kordofan state, Blue Nile state and Abyei. But the precise border between north and south remains undefined, there is no agreed local government for the area and there is a lack of transparency on oil revenues.

In May 2008 clashes in Abyei between Sudanese armed forces and the SPLM displaced thousands of civilians and underlined the ongoing tensions in the area.

Control over individual oilfields is also disputed. Southerners and northerners both say the large Heglig oilfield in Unity state falls within their territory.

Then there are the oil contracts. Before and after the 2005 peace agreement, the SPLM signed deals to develop oil concessions in the south, which the government has since contested.

All these unresolved issues could delay the flow of oil revenues and development in general, increasing the possibility of renewed conflict in the future.