Shaka Zulu, the 19th-century warrior king dubbed Africa's Napoleon, was not the bloodthirsty military genius of historical depiction, says new research.

His reputation for brutality was concocted by biased colonial-era white chroniclers and unreliable Zulu storytellers who turned the man into a myth.

There is scant evidence for cherished beliefs that Shaka was illegitimate, bullied as a child and that he invented a new form of warfare which allowed the Zulus to conquer swaths of southern Africa, according to Dan Wylie, an academic at South Africa's Rhodes University.

Dr Wylie described his book, Myth of Iron: Shaka in History, as an anti-biography because the material for a trustworthy biography did not exist. "There is a great deal that we do not know, and never will know," he said.

What is known is that Shaka, born into the small Zulu clan that formed part of the Nguni tribes, engaged in conflicts that killed and displaced hundreds of thousands of people. By the time he was assassinated by his half-brothers in 1828, on the eve of Boer and British incursions, Shaka reigned over much of present-day Zimbabwe, Swaziland and South Africa.

Shaka is often compared to Napoleon but Dr Wylie said although the African may have refined military innovations, he did not invent them. The author also doubted accounts that Shaka was illegitimate and bullied as a child, claims that some historians treated as fact. Dr Wylie detected numerous contradictions in the oral history of Zulu storytellers. Worse, the academic found that colonial-era white writers distorted and exaggerated the meagre historical record to turn Shaka into a despotic monster.

Nathaniel Isaacs, who wrote about Shaka in Travels and Adventures in Eastern Africa, published in 1836, wrote to a fellow author, Henry Francis Fynn, advising him to smear Shaka and his successor, Dingane: "Make them out to be as bloodthirsty as you can and endeavour to give an estimation of the number of people they have murdered during their reign[s]." This would help sell Fynn's book and encourage British annexation of Zulu lands, which would mean a "fortune" for both authors. Dr Wylie said this had set the tone for future distortions, such as the 1980s television series Shaka Zulu, starring Henry Cele.