“Africans have killed Arabs for years over grievances about land and water,” said Musa Hilal, a notorious strongman among the Janjaweed, while giving interviews over tea and cakes in a Khartoum hotel. “Things like that give birth to bitterness. When the government put forward a programme of arming all the people, I will not deny I called our sons and told them to become armed, and our sons acquiesced.”

Hilal, 43, believes his job as a leader is to protect the Arab people of Darfur and their honour. It’s a job that has led to two prison sentences. The leader of an Arab clan that had been trying to take African lands since the 1970s, he was jailed for killing 17 Africans in Darfur. Years earlier he had also been imprisoned for robbing a bank.

Now, though, he is a free man, and his work has been officially sanctioned. He was released from prison to help reorganise the traditional Janjaweed Arab militias as a government- supported fighting force to put down the rebellion of African people in Darfur.

However, yesterday a UN report blamed Sudan’s government for extra-judicial killings, amounting to crimes against humanity.

Hilal’s militias were authorised by President Omar al-Bashir to burn villages and loot livestock and food on a grand scale. They were also allowed to rape with impunity, according to human rights groups. They are now also busily settling in cleared African villages.

The president enlisted the help of the Janjaweed not just because of Arab kinship but because they are exceptionally hard men. It is far better to have them on your side than against you, says Shaun O’Fahey, who lived among the Darfur Arab nomads and who is now Professor of African History at the University of Bergen, Norway.

“They [the forebears of the Janjaweed] immigrated 12 centuries ago from the deserts of the Arabian Peninsula,’’ says O’Fahey. ‘‘Their hospitality is traditionally Arab. When I went to nomadic encampments I was received as an honoured guest, although conversation was limited, mainly about religion.

“The situation really disintegrated with the decision of the Sudanese prime minister in the mid-1980s, Sadiq al-Mahdi, to give arms to the Arabic-speaking nomads, ostensibly to defend themselves against the Sudan People’s Liberation Army [in far distant southern Sudan]. Nobody was surprised when they began to turn their guns on their African farming neighbours. Conflicts over wells that in earlier times had been settled with spears or mediation became much more intractable in an area awash with guns.”

The roots of the Janjaweed, an Arabic colloquialism which translates as “a djinn [devil] on horseback with a gun”, can be traced back 1200 years, when Arab and African cultures began meeting, mixing and clashing on a great human fault line that runs for thousands of miles across Africa on the southern margins of the Sahara Desert.

During those centuries the Arabs have largely Islamicised the Africans, taking countless hundreds of thousands of them as slaves, a trade that continues to this day. While there has been much miscegenation, those who class themselves as “Arabs” consider themselves racially and culturally superior to the “Africans”.

In Darfur, the vast western region of Sudan which is bigger than Britain and the location of the world’s biggest contemporary humanitarian crisis, the Janjaweed militiamen are primarily members of nomadic “Arab” tribes who have long been at odds with Darfur’s settled “African” farmers, who tend to be darker skinned.

Until last year, the conflicts were mostly over Darfur’s scarce water and land resources. The best land is in the Jebel Marra, an 8000-square mile area of mountains rising to more than 10,000 feet in the central belt of Darfur, and which no aid agencies or journalists have reached during the present crisis.

The Jebel Marra massif, unlike the surrounding semi-desert plains, has rich, volcanic soils laced with lush, forested river valleys dotted with waterfalls. It is the home of the region’s biggest black African tribe, the Fur (Darfur translates as “abode of the Fur”), and several smaller African tribes, including the Zaghawa and the Masalit.

One of the few descriptions of the Jebel Marra was written by Peter Abbott, the British colonial medical officer for Darfur in the 1940s. “The contrast with the rest of Darfur was dramatic,” wrote Abbott, who tried unsuccessfully to shoot leopards that were carrying off the goats of Fur farmers.

“The cool climate and high rainfall supported springs and streams and lush pastures with a flora not unlike the Mediterranean coastland. I was able to see all kinds of colourful birds unknown in the rest of the Sudan. The air at my camp beside a volcanic lake at 10,100 feet was bitterly cold and my camp furniture and bed were arranged around a huge fire.”

Meanwhile, on the Darfur plains below the mountains, the explorer Wilfred Thesiger, then an assistant district commissioner, was slaughtering lions almost for the fun of it, often hiring Fur trackers from villages on the margin of the plains and mountains.

The water and soil resources of the Fur, Zaghawa and Masalit became targets for the post-independence Sudanese government, through its Janjaweed surrogates. Unusually severe drought, desertification and over-population on Darfur’s plains put the nomadic Arab tribes under severe stress from the early 1980s onwards. While there had always been localised skirmishes with Africans at the height of the dry season, when the Arabs moved their camel and goat herds into the Jebel Marra foothills, there has been a systematic drive since 1985 by the nomads to occupy permanently stretches of African land.

Before 1985 the skirmishes were largely spontaneous and of low intensity, settled by local negotiation. Since then the conflict has grown ever more intense.

“The nomadic scramble into the rich agricultural central heartland is the cause of the continuing conflict,” says Dr Mohamed Suliman, a Sudanese academic at the Swiss Institute for Conflict Resolution. “It is the contest of the drought-stricken for the green oasis.

“Whatever the perception of the Darfur conflict, it is one that is being fought primarily over the control of a thriving resource base in the middle of a zone of scarcity. It is a classic ecological conflict.”

Suliman documents many serious Arab-African clashes in the past 20 years in Darfur, far from the nearest inter national news agency bureau, far from the centre of Sudanese power and accessible over land from the Sudan heartland in the Nile Valley only by shifting tracks through the savannah and desert.

In this cat’s cradle of conflict, a powerful role has been played by the shadowy and powerful Jellaba, the northern Arab Sudanese merchant class, which historically was responsible for the slave trade. The Jellaba made great fortunes by introducing large-scale mechanised farming into central and southern Sudan. The farming destroyed fragile soils, but the Jellaba now had international links as extractors and sellers of raw materials. They formed alliances in the 1980s with the Janjaweed, who were seizing cattle from black tribes for export by the Jellaba to Libya, Syria and Jordan.

While recent journalistic accounts suggest that the current Darfur crisis began only this year, serious conflict has been continuous there since 1985, at the height of a particularly serious drought that ravaged the region. Triumphant nomads, accounting for only 15% of the Darfur population, backed by the Sudan government and the Jellaba, roamed Darfur “liberating” land and driving black farmers from their homes.

The conflict escalated after the military coup of 1989 which brought to power the hardline Arab and Islamic nationalist government of President al-Bashir, who gave huge road-building and construction contracts to Osama bin Laden. O’Fahey says: “The coming to power of the National Islamic Front [NIF] injected an ideological and racist dimension to the conflict, with the sides clearly defining themselves as ‘Arab’ or ‘Zurq’ [black].

“My impression is that many of the racist attitudes traditionally directed toward slaves have been redirected to the sedentary African communities.”

The developing Darfur war was ratcheted up much further when the African people of Darfur decided to fight back through two liberation movements, the Justice and Equality Movement (JEM) and the Sudan Liberation Army (SLA). In early 2003 these rebels attacked a military garrison in Darfur, destroying four helicopter gunships, two Antonov aircraft and, according to government officials, 75 Sudan Army soldiers.

At the same time the NIF was negotiating a settlement in a separate conflict, the country’s 50-year civil war in the far south of Sudan between the government and animist and Christian Africans. President al-Bashir wanted to send a strong message to several other rebellious parts of the country, the largest in Africa, that there would be “no surrender” by Khartoum.

That message has led to what is now generally regarded as genocide and it may get even worse. O’Fahey believes that now the Janjaweed genie is fully out of the bottle, it will be almost impossible for the Khartoum government to put it back again, even if it wished to and despite the international community’s demands that it do so.

“The Janjaweed were always very tough,’’ says O’Fahey. ‘‘They travel long distances on camels with their horses tethered behind. They mount their horses only to wage war. If they don’t want to be found, nobody will be able to find them. Darfur is huge.”

But the professor has no doubt either about the Janjaweed ideology. “It’s essentially racist. They have contempt for Darfur Africans and do not believe they are true Muslims. The genocide in Darfur will be very hard to end even if there is will among the international community to do so.”