A single look at the outlandish portrait photograph, taken in 1864, of ‘Colonel’ Alexander Gardner, one of the most enigmatic adventurers of the 19th-century Great Game in Central Asia, should be enough to transfix anyone. Straight-backed and clutching a sabre, he is clad in tartan trousers, a tartan jacket and a tartan turban finished with a regal plume of egret feathers. A huge beard conceals a long-unhealed sword slash across his throat, which he had to secure with metal clamps whenever he wanted to eat or drink. His gaze is haunted but steely, as if he were well accustomed to dealing with horror.

John Keay, one of this generation’s greatest authorities on exploration in Asia, has produced a fascinating biography of this strange and contentious figure. The sources about Gardner are fragmentary and disputed, but Keay has masterfully drawn them together to produce an elegant and convincing account.

Gardner, it seems, was an American citizen of Scottish descent, born in 1785. In his youth he spent some time in Ireland, where he picked up a smattering of naval gunnery. He then joined an elder brother who was an engineer in the Russian city of Astrakhan. When his brother died in 1817, Gardner decided to continue eastwards and seek his fortune in the wilds of Central Asia.

This was not a completely untrodden path. A number of European military officers had headed east after the Napoleonic Wars and had been rewarded with gold, estates and harems by local potentates for modernising their armies. Maharaja Ranjit Singh, founder and ruler of the Sikh Empire of the Punjab, was particularly generous in this regard. However, those who crossed Central Asia were taking a serious risk. The ways were long and uncharted. Famine, disease and banditry, not to mention torture and execution by capricious khans, were rife.

Gardner quickly became used to these dangers. He travelled at first disguised as an Arab, passing between caravans and bands of freebooters. On one occasion he was captured and nearly sold into slavery, though his wanderings near Merv, which he glosses over in his accounts, are likely themselves to have been slave-raiding expeditions, then endemic around northeast Persia. He learned how to kill on the road when it was a necessity. Yet he could also use his considerable charm and astuteness to preserve himself. On being arrested by the Khivan authorities, who suspected him of being a Russian spy, he won over the khan by revealing his American identity, assuring them he was no threat to Khiva’s independence.

Around 1823, he reached Afghanistan. The country was then a patchwork of dominions ruled over by warring interrelated families. His charm won him employment under Habib-ulla Khan, a nephew of Dost Mohamed, the ruler of Kabul who was later deposed by the British in the First Afghan War. Habib-ulla himself had been ousted by Dost Mohamed from the Kabul throne, and he was now waging a guerrilla war against his uncle from a stronghold north of Kabul. Gardner was put in charge of 180 cavalry and ordered to plunder the caravans intended for the capital. Before long, he captured a convoy carrying one of Dost Mohamed’s wives and other female members of the royal household. One of them was given to Gardner as a reward and bore him a son.

Gardner’s domestic bliss was shattered a couple of years later when Dost Mohamed’s soldiers raided his compound and murdered his young Afghan family. Gardner fled, passing through the Pamirs, Xinjiang and Kashmir. He was the first European to visit Kafiristan in eastern Afghanistan, the remote region of the Hindu Kush where the native peoples clung to ancient animist beliefs, still untouched by Muslim proselytisation. Eventually he found service with Ranjit Singh as an artillery officer. However, his remit ended up being wider than gunnery. The Sikh Empire collapsed into vicious internecine conflict after Ranjit’s death in 1839. Gardner undertook the dirty work of various belligerents, parading severed heads on poles and, most notoriously, cutting off the ears, nose and fingers of a Brahmin magnate. Keay’s handling of this difficult period of history is brilliantly lucid.

Unsurprisingly, Gardner’s reputation has been controversial. During his retirement in Kashmir in the more peaceful 1860s he mesmerised visitors with tales of his travels, using a collection of native hats as props to illustrate different characters. He seemed to them a hoary monument from the heroic age of travel. He might even have inspired Kipling’s The Man Who Would Be King.

Yet many sought to undermine his stories. To some in Britain, Gardner was an uncomfortable figure. He was frequently critical of British policy and might even have seen active service against the British when they invaded the Punjab in the 1840s. He lived for many years in the guise of a Muslim and was vocal against Western missionaries: ‘On starving or hungry stomachs … dry tracts and Bibbles [sic] are not always the things that sit best,’ he once observed. The fact that he was supremely careless about accurately and systematically recording his travels made him an easy target for debunkers. Soon after his death, detractors working through Indian archives claimed that he was in fact an Irish deserter from the Royal Navy and that his pioneering journeys to Kafiristan and Central Asia were all fantasies.

For many years, Keay was a sceptic. He omitted Gardner from The Royal Geographical Society’s History of World Exploration, which he edited in 1991. However, this gripping new book is a much-needed reassessment of the evidence in Gardner’s favour, including a wealth of recently rediscovered archival material. Gardner will never be an easy figure for a modern audience, given his violence and his brushes with the indigenous slave trade, not to mention the inexact chronicling of his travels. However, the fact that, unlike many other 19th-century travellers in the region, he managed to make his extraordinary journeys unencumbered by Western governmental support and Western cultural assumptions means that he should not be ignored. John Keay’s biography restores him to the place in history that he should justly hold.