Early in this readable, rigorous and important account of the tragedy of the world’s youngest nation, Peter Martell, a former BBC correspondent, explores the stories that the people of South Sudan have told to one another about their shared heritage.

These “fireside fables” have been handed down through the generations: accounts of war, slavery, violence, defiance and rebellion that were instrumental in giving the hugely diverse communities of South Sudan something approximating to a shared identity. Above all, they have reinforced the sense that they are people who have long been victims.

That historical suffering has been at the heart of these stories is entirely justified. The early chapters of First Raise a Flag cover the raids by heavily armed and organised slave traders who came down the Nile from the north in the 19th century to capture tens of thousands of men, women and children for sale into markets in the Middle East. There followed the far-from-benign neglect of British administrators who deliberately underdeveloped the south of Sudan. When the imperialists left in 1956, their rule was replaced by that of a political elite in Khartoum that displayed systematic brutality, authoritarianism and discrimination, irrespective of its ideological orientation.

That so many of those living in the lands beyond the Sudd, the vast swamp barring the upper course of the Nile and dividing the broadly Christian and “African” south of Sudan from the largely Arab and Muslim northern parts, might wish for independence after such appalling treatment by others is entirely understandable. The sacrifices made to achieve this aim were huge: up to 3 million people may have died in the successive wars fought by the southern Sudanese against Khartoum between 1956 and 2005, when a peace deal was finally concluded.

Many can be blamed, but standing above them all are the new country’s venal, corrupt, brutal and brutalised leaders

Though a bewildering array of local, regional and international actors – Ethiopia, Uganda, Eritrea, Libya, Cuba, Israel, the USSR – exploited the conflict with astonishing cynicism, there was rarely any sustained attention devoted to one of Africa’s longest wars by western media, policymakers or even humanitarians. When such interest eventually came, faith and blind optimism were more evident than deep consideration of how to achieve successful outcomes for the many millions whose lives should have mattered most. In 2011, with enthusiastic backing from the US and, especially, African American and conservative Christian lobbies (and George Clooney), South Sudan came into being, free at last.

Martell, a BBC reporter based in Juba, the new country’s capital, was there to witness the optimism and joy that greeted independence. His experience, gained over years of living in and reporting on the country, is invaluable and notably absent from many other accounts.

Historical narrative and careful analysis are thus mixed with interviews with individuals chosen to illustrate the broader story. Each draws a new portrait. Martell is a sympathetic and sensitive listener and his writing powerful and moving. We hear the voices of those who have fought, fled, struggled, hoped and suffered; we see both the celebrations and the skeletons.

There are few of the former, many of the latter. Martell carefully and accurately describes what has happened to South Sudan since 2011: a tragedy. There are many who can be blamed, but standing above them all are the new country’s venal, corrupt, brutal and brutalised leaders. These rapidly set about the systematic looting of billions of dollars from oil revenues, then unleashed armed men on civilians to rape, mutilate, burn, torture and kill on a horrific scale. As many as 400,000 have died in a civil war that, if currently suspended by a precarious truce, still threatens to become a genocide. This is a death toll to rival that in Syria. Millions have fled. Famine and cholera are killing daily. The country is a ruin.

On a reporting trip to Pibor, in the east of South Sudan, last year, I found misery of a depth I have rarely seen in 20 years of working in such places. The international community, with all its experts, peacekeepers, humanitarians and fine rhetoric, has proved singularly incapable of stopping atrocity upon atrocity.

None of this makes easy reading and Martell does not flinch from the details. But he also carefully explains why it has happened. The account of divisions within the ranks of the insurgent forces before independence is particularly useful and goes a long way to explaining the horror of today. The 1990s saw internecine violence on increasingly ethnic lines, with consequent famine and mass displacement. Martell rightly wonders why anyone helping create South Sudan would have thought there would be any improvement once the common enemy of Khartoum was removed.

There is little reason to be hopeful, though Martell does his best. Running through First Raise a Flag are references to storytelling: the fables told around the fireside, the narratives of witnesses and victims, the simplified bulletins the author broadcasts in his dispatches for the BBC, the propaganda of regimes and campaign groups, the lies of corrupt commanders.

“The stories of long ago were once themselves repeated and reinforced by new rounds of violence,” he writes. “In an oral culture, exact dates slip. History as a linear narrative is distorted, because stories are as alive now as they ever have been. These are not the dry, dull dates of faded textbooks. They are the events that had shaped life and have been fused into the history with blood.”

• First Raise a Flag: How South Sudan Won the Longest War but Lost the Peace by Peter Martell is published by Hurst (£25). To order a copy for £25 go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99