On 9 July 2011, at the stroke of midnight, the world’s newest state – the 193rd member of the United Nations – was born amid jubilation and a wave of optimism after five decades of titanic, catastrophic struggle had left two million or more people dead.

Fireworks went off in Juba, the ramshackle capital of South Sudan. Church bells rang while people of the new nation, singing and beating drums, packed the streets. A huge specially built independence clock tower shone out a digital display in red letters, “FREE AT LAST” – after half a century of tyrannical rule from Khartoum by an Islamic government in the former unified state of Sudan.

Hollywood stars like George Clooney, Mia Farrow and Don Cheadle, along with some foreign journalists and human rights activists, portrayed South Sudan as a bucolic land now about to be ruled by freedom fighters inspired by high ideals. Managed properly, they trilled, South Sudan – its fertile soils fed by the mighty waters of the White Nile that bisects the country, and with the help of generous foreign aid – the new nation could become the breadbasket of Africa.

But last week an articulate 15-year-old South Sudanese schoolgirl, her name given only as Monica, shattered the illusion, telling BBC special correspondent Tim Franks: “The truth about South Sudan is that this country is rich (only) in killing people.

“I’m losing hope for my country. Our elders just hate each other.”

South Sudan, so very soon after its birth, is riven by a terrible political and tribal war that has received scant international attention from a modern world plagued by conflicts and hatreds elsewhere.

“This war is more dangerous than the war with the North (Khartoum). This is war between brothers,” said one man in an extraordinary report just published by the African Union (AU).

Extraordinary? Remarkable because in the past the AU has vigorously criticised non-Africans who dared to highlight eruptions of barbarism and tribalism on the African continent. But now, in its 315-page report with the low-key title of AU Commission of Inquiry on South Sudan, the AU itself has documented despairing levels of atrocities, tribal enmities and political irresponsibility in their continent’s newest state. This marks a historic sea change in Africa’s attitude towards its own travails, and is arguably a healthy development.

South Sudan’s civil war, unlike its previous wars with the Moslems of the north, has not become a cause célèbre. For most of the world the AU report is probably a first glimpse inside the conflict – and the glimpse is dreadful.

The violence depicted is far beyond anything that South Sudan has ever seen – including in its half century fighting the North – and describes some of the worst atrocities witnessed in Africa since the 1994 Rwandan genocide.

Gang rape, the massacre of civilians, sometimes in churches, and instances of captives being forced to jump into bonfires are among a long list of evils committed during South Sudan’s intensifying civil war, says the report, written more than a year ago but held back until now because officials feared that it would derail on-again, off-again peace efforts.

According to the investigation, the South Sudan government of President Salva Kiir and rebels led by Riek Machar, the Vice President he sacked in the summer of 2013, have both targeted civilians.

Perhaps the report’s most shocking claim is that fighters have forced survivors to eat the flesh of charred bodies and drink the blood of slaughtered friends and relatives. There is more than a whiff here of the “The horror, the horror”, the dying words of Kurtz, surrounded by the heads of slaughtered Congolese on poles, in Joseph Conrad’s Heart of Darkness.

The conflict’s origins are mired inevitably in a complex history. Following Lord Kitchener’s1896-1898 conquests, in battles at Ondurman, where an army of Sudanese Dervishes was massacred, and elsewhere, Britain governed Sudan – five times the area of the UK – with just 130 white colonial civil servants, members of the elite Sudan Political Service. Only 400 people ever served in the body from the beginning to the end of the British era.

From 1924 onwards, the British divided Sudan into two separate territories – a predominantly Moslem Arabic-speaking north, and a predominantly black African Animist and Christian south, where the use of English was encouraged among some 60 tribal groups speaking 80 different ethnic tongues.

While trying to control almost half the world at the time, the British instituted in Sudan a classic “divide and rule” strategy to maintain stability. Development was concentrated in the Moslem north. Only a handful of Africans from the south entered the national civil service. (A mere six of 800 Sudanese civil servants at independence in 1956 were southerners). Britain reasoned that the African pastoralists in the “primitive” south were not ready to open up to the modern world – unlike the northerners.

The British encouraged Islamisation of the north with financial help to build mosques and to subsidise pilgrimages to Mecca. In southern Sudan, however, they wanted, with the help of Christian missionaries, to prevent the spread of Islam and preserve what they described as the “pure African way of life of the southern people.”

The British policy document for the south stated an intention to rule through chiefs and “build up a series of self-contained tribal units with structure and organisation based upon indigenous customs, traditions, and beliefs." These southern tribal units were to be completely separated from the rest of the country. Northern officials were transferred out of the south; trading permits for northerners were withdrawn; and speaking Arabic and wearing of Arabic clothing were discouraged.

Little wonder then that a southern insurgency was forming before British withdrawal and then escalated into a full-scale war between North and South, in the shape of South Sudan Liberation Army (SSLA). The first sixteen years of war ended only in 1972 under an agreement in which Khartoum granted significant autonomy to the South under a federal arrangement rather than outright secession demanded by SSLA.

The 1973 Southern Constitution, while establishing Islamic Law and custom as the main sources of law, protected Christianity and decreed that personal matters of non-Moslems would be regulated by African custom.

North-South war erupted again in 1983 when Khartoum abrogated the 1972 agreement, imposing the use of Arabic in the south together with Sharia law, involving executions and amputations, while dissolving the South’s Regional Assembly and weakening by various measures the hold of the south’s largest tribe, the Dinka. Khartoum thus introduced a new dynamic into the North-South war – a South-South conflict between the Dinka and the second largest tribe, the Nuer, within the “united” anti-Khartoum SSLA that reverberates to this day. Other large tribes, including the Shillock and Azande, were sucked into the internal conflict.

In 1991 a Nuer faction of the SSLA, led by Riek Machar, massacred a large number of Nuer in the White Nile town of Bor, 130 miles north of Juba. Amnesty International said more than 2,000 Dinka were killed, and the human rights organisation estimated that at least 100,000 people fled the area. Famine followed the massacre as Machar’s Nuer looted and burnt villages and raided cattle.

Some 25,000 more people died as a result of hunger, according to Amnesty. At the time, Machar described the incident as “propaganda” and “myth” despite evidence of mass killing shown by bones and corpses in the aftermath. In 2012 he admitted responsibility.

Incidents like that at Bor meant the South remained riddled with internal tensions by the time in 2005 that Khartoum conceded an independence referendum for the southerners. More than 99 percent of those who voted opted for complete independence, which came in July 2011. Past internal differences between the southerners seemed to have been overcome when Salva Kiir, a Dinka who wears black cowboy hats, became President with Machar as his Vice President.

But the title of the acclaimed post-colonial novel Things Fall Apart by Nigerian author Chinua Achebe could not have been more applicable to what happened next between the two top leaders of “liberated” South Sudan.

At the end of 2013, on 15 December, a fight broke out in Juba between Dinka and Nuer soldiers in Salva Kiir’s Presidential Guard. Kiir accused his deputy of attempting a coup. Machar denied it, but fled to the bush, demanding that Kiir resign. Inter-tribal fighting spread with incredible rapidity, engulfing entire swathes of the young nation.

In the oil-rich, swampy Upper Nile province the combatants fought full-scale tank battles.

Civilians died in tens of thousands: hundreds of thousands fled into refugee camps, run by United Nations “peace keepers” and divided strictly according to tribe. Entire towns have been wiped out, including Malakal, South Sudan’s second city and capital of Upper Nile.

Malakal, in the heart of rich oilfields, has changed hands more than ten times in the past 22 months. The once thriving city, where tribes met, traded and prospered, lies empty, its spanking new children’s hospital, built with international help for independence, burnt to the ground and littered with rusting bedsteads. A nearby International Red Cross building has been totally looted.

The depravity sank to a particularly cruel depth in May this year when Kiir’s government forces killed 129 children in just three weeks of mayhem in Unity State. Survivors said boys had been castrated and left to bleed to death so they would not live to seek revenge. Girls as young as eight were gang-raped and others were thrown alive into burning buildings. An estimated 15,000 child soldiers, of whom some 1,500 have died, have been recruited by the opposing groups.

These are scenes repeated in countless centres across a country so vast and sparsely inhabited that it is home to one of the world’s greatest wild animal migrations of more than a million kob and tiang antelopes between the savannahs of the east and their dry season refuge in the Sudd swamps of the White Nile.

The AU report calls for an “Africa led, Africa owned, Africa resourced” court to prosecute the country’s war crimes and crimes against humanity. The call for such a court comes against the background of African opposition to the International Criminal Court, based in The Hague, which has been dubbed anti-African because it has focused almost wholly on indictments of African figures, including Sudan’s President Omar al Bashir.

It is into this maelstrom that David Cameron last month pledged to send 300 British armed forces to reinforce 11,350 other United Nations peacekeepers. Since there is no peace to keep, UN troops are involved mainly in protecting refugee camps to which some quarter million people have fled for protection.

Another two million of South Sudan’s 11.5 million people are estimated to be internal refugees, living under trees and elsewhere.

It is unclear when the British military contingent will leave for South Sudan, but President Kiir’s government has promised them an unfriendly welcome. His foreign and international cooperation minister, Dr Barnaba Marial Benjamin Bil, is “positively hostile” to the impending British arrival, according to one Western diplomat who added: “South Sudan does not and should not have a say on who contributes to the UN peacekeeping mission.”