The death of John Garang, leader of the Sudan People’s Liberation Movement (SPLM) for the last 21 years, has upset US-led efforts to secure the country’s huge oil-producing potential.

Garang was sworn in as vice president of Sudan only three weeks ago on July 9. He joined the government of his old enemy, President Omar Hassan al-Bashir, ending Africa’s longest running civil war.

Garang was killed at the weekend when his helicopter crashed in the mountainous border region between Sudan and Uganda. His death immediately provoked disturbances in Khartoum, the capital of Sudan. Riots broke out as southern Sudanese blamed the government for his death and clashed with northerners.

At least 84 people have been killed and 100 are in hospital after the police opened fire. Military helicopters were reported to be flying over the capital and armoured vehicles took up position at strategic points as a curfew was imposed.

The New York Times declared the death a “tragedy” and “heartbreaking.”

Regional and world leaders have called for calm and insisted that the peace deal will go ahead. But the market response suggests that their optimism is not widespread. Shares in White Nile, the company that recently negotiated oil concessions with Garang, fell by 13 percent when the news broke. US Secretary of State Condoleezza Rice has sent two top envoys “to maintain momentum” in the north-south peace deal and to insist that the government press on with reaching an agreement in Darfur.

The crash took place as Garang was traveling in a Ugandan military helicopter back to the SPLM’s base at New Site in southern Sudan after visiting President Yoweri Museveni of Uganda. Its cause is unknown. It may have been an accident resulting from bad weather conditions. There are suggestions that the helicopter ran out of fuel.

It is also possible that the crash was a deliberate act of sabotage. Garang had many enemies both in the Khartoum government and in the south of Sudan, where he has imprisoned and killed many SPLM opponents. But whatever the cause, Garang’s death will destabilize the already tenuous US-backed southern peace deal. It also ends hopes that Garang would broker a deal in the western Darfur region. Conflict is also simmering in the eastern parts of Sudan.

The Royal African Society’s director, Richard Dowden, summed up the situation. “The danger is that Garang’s death will split the Sudan People’s Liberation Movement,” Dowden told the Guardian.

He continued: “Garang was an old-fashioned dictator. There’s no way his was a consensual leadership. He didn’t like successors. He fell out with the most likely of them and it’s unclear who’ll take over. A large number of people who hated Garang may say, ‘Here’s our chance.’ The north is likely to sit back and snigger.”

Garang, who was 60 years old when he died, was a member of the Dinka tribe, the largest tribal group in Sudan. He studied at Grinnell College in Iowa and went on to receive a PhD in economics from Iowa State University for research on the agricultural development of southern Sudan. He was part of the Anya Nya movement that attempted a southern breakaway from Khartoum in Sudan’s first civil war.

After a peace deal was signed in 1972, he was among former rebels who joined the Sudanese army. He rose to the rank of lieutenant colonel and returned to the US to take a commander’s course at Fort Benning, Georgia. In 1983, he was sent to crush a mutiny by officers in his home district after President Nimeiry imposed sharia law. Instead, he joined the rebellion, which began Sudan’s second civil war, and ultimately emerged as the head of the SPLM.

The roots of the repeated civil wars that have characterized the history of Sudan since it became independent in 1956 can be traced to the legacy of tribal, religious and cultural divisions that British colonial rule left in its wake.

Britain always ruled north and south separately and sought to foster rivalries between Arabs and Africans, Christians and Muslims, as well as between the many tribal groups. These long-standing antagonisms were exacerbated by Cold War rivalries between the US and the Soviet Union and by US sponsorship of Islamic fundamentalism. More recently, Sudan’s oil has become the attraction for rival foreign powers.

Sudan’s President Gaafar al Nimeiry came to power in 1969 with the backing of the Sudanese Communist Party, and was initially supported by the Soviet Union, even when he imprisoned and executed Communist Party members. From 1971, Nimeiry adopted an increasingly pro-Western policy.

In 1983, he introduced Islamic sharia law in response to popular protests about the imposition of International Monetary Fund austerity measures. This provoked an uprising in the south, where the majority of the population is Christian or follows traditional African religions.

The southern rebels were supported by the Soviet-backed Ethiopian Derg regime of Mengistu Haile Mariam that overthrew the pro-Western regime of Emperor Haile Selassie in 1974. Two million people are said to have died in the civil war that followed. Nimeiry was overthrown in 1985.

Ultimately, in 1989, the National Islamic Front government of Omar Hassan al-Bashir emerged in Khartoum. American backing for Islamic fundamentalism in Afghanistan had done much to sustain this reactionary ideology in Sudan, but with the end of the Cold War US policy shifted.

Following the breakup of the Soviet Union and the fall of the Derg in 1991, the US threw its support behind the southern rebels. Garang always showed himself to be a pragmatist with no firm ideological conviction, who readily shifted his allegiance.

The US declared Sudan to be a state that supported terrorism and began to channel covert military aid to Garang’s SPLM though Uganda, as well as $2.13 billion openly in aid to the SPLM-held areas of the south. In 1998, the US bombed what it claimed was a chemical weapons factory in Khartoum, but was, in fact, a pharmaceutical plant. Ignoring SPLM human rights abuses, the US House of Representatives condemned the Sudan government for its “genocidal war in southern Sudan ... and continued human rights violations.”

More recently, Washington’s policy towards Sudan has shifted again. As the Bush administration prepared for war with Iraq, it modified its attitude to the Sudanese government, which agreed to cooperate in Washington’s “war on terror,” handed over 30 people who were said to be associates of Osama bin Laden, and welcomed US intelligence agencies to Khartoum. In return, the US allowed restrictions on Sudan’s access to international finance to be lifted, making foreign investment in the country easier.

US oil companies were also bringing pressure to bear on Washington. In 1998, a 1,600-mile pipeline was opened between Unity State in the south of Sudan and the port of Beshair on the Red Sea. Efforts by the SPLM to blow it up failed.

The pipeline has the capacity to carry 250,000 barrels of oil a day. There are thought to be more than two billion barrels of oil reserves in Sudan.

US companies were effectively excluded from Sudan, leaving the field open to European and Asian rivals, especially China, which has invested heavily in Sudanese oil production.

Even had Garang survived, there would have been continued conflict at a diplomatic level, which always threaten to flare up into military clashes. With Garang gone, the prospects for open military conflict are considerably greater, but his death may mean that inter-factional rivalries in the south temporarily assume more importance than the north-south conflict.

Khartoum will undoubtedly use such antagonisms to extend its hold over the oilfields. It has long relied on the Southern Sudanese Defence Force to carry out attacks on the civilian population in an attempt to clear them from oil-rich areas.

Before Garang’s death, the Brussels-based International Crisis Group (ICG) had already drawn attention to the precarious nature of the north-south peace deal that had been agreed under US auspices. The ICG published a report in July which pointed to the failure of the Khartoum government to implement aspects of the peace deal and the inability of the SPLM to make the transition to government.

The report suggested that the oil contracts which Garang signed shortly before he died contravened the peace deal and should be scrapped. It called on the US and the UK to step in and immediately determine boundaries between Khartoum and the SPLM in the oilfields. Underlying this demand is the rivalry between Europe, China and the US over Sudan’s oil and control of a country that is strategically placed between Africa and the Middle East on the shores of the Red Sea, one of the world’s great shipping lanes.

Another significant factor in Sudan’s precariousness is the massive social crisis across the whole of Africa, which has been plunged into permanent poverty by the policies of Western imperialism.

In Darfur, an estimated 2 million people have fled their homes. An unknown number, certainly more than the 70,000 estimated by the United Nations, have died from hunger, disease and violence. In the south of the country, after years of civil war, there is no infrastructure and tens of thousands of people are displaced. In Khartoum, refugees from the south live alongside impoverished northerners in shanty towns that were the scene of the recent rioting.

As in the rest of the continent, the different ethnic, religious and tribal groups have been pitted against one another in brutal wars and communal violence by elites that are attempting to further their own interests, with imperialist backing.