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In the 19th century, geographic exploration was the same mixture of science and political ballyhoo as a contemporary space shot. The Mekong Exploration Commission (MEC) of 1866-68 was France's bid to be one of the "civilised" nations shedding light on the planet's dark places. It deliberately exceeded the fashionable British expeditions through Africa in duration, scope and ambition. Science was an umbrella under which the blatant colours of the flag of empire could be sheltered and camouflaged.

John Keay is a literate traveller who, in previous works, has walked the ground of most of Britain's involvement with the East. Here, he charts the invention and deployment of the concept of Indo-China. Yet his claim that the expedition is forgotten because the French do not honour explorers or their colonial past is untrue. Those in Africa, even the most ethically doubtful, receive the full panoply of commemoration, and if their empire is not more memorialised, it is only because the French have never really given it up.

Nevertheless, Keay has done the job with ruthless determination, followed in their footsteps through jungle and swamp, plumbed the depths of 150 years of geography, history, politics and economics and even braved the Bibliothequ Nationale, one of the most inhospitable terrains on the planet, to try to understand this project and its effects. The account of the journey becomes a thread that links pearls of wisdom, ancient and modern, about everything from the bizarre tidal forces of the Mekong to the curious and hazardous semi-sovereign state called 9/11 that exports buffalos and drugs in equal measure. As Keay shows with wit and erudition, the Mekong has somehow always resisted regularisation by maps and national borders, constantly shifted its ground and cultivated ethnic and cultural ambiguity and anomaly.

The 20-strong MEC set off from the odd rump that was France's freshly seized Saigon enclave and followed the river through free Annam to its new protectorate of Cambodia. Angkor Wat was at that time included in Thailand, but the expedition made a diversion to it and it would be part of its remit to "reclaim" it for Cambodia in the name of France's role as heir to the dismembered Khmer Kingdom. Following its passage, similar arguments would be used elsewhere to adjust colonial boundaries of Laos, Burma and China while, in the south, Britain played the same game with what are now the northern provinces of Malaysia but defended the core of Thailand as a barrier against further Gallic incursion.

From the start there was a clash of personalities, with the easy-going leader, Doudart de Lagree, being constantly dominated by the lean and hungry surveyor, Francis Garnier. The account of the journey, in ever-smaller boats and under ever-worsening conditions, over more than 11,000km, is a tale of obsession, heroism and the attempt to translate a myth of paradise into an enlightenment map. Insuperable barriers to navigation are "disappeared" in a crazed attempt to make it the commercial artery of France's lunge for the heart of Asia. Wild beasts, bronze maidens, dragons, gold mines and malaria daze and beguile the Westerners into delirium. When the emaciated, depleted, diseased expedition makes its way into China and all useful information has been gained, Garnier insists on making another demanding trip into rebel-held territory to see the divine river once more, and so leads Lagree to his death.

Like all the expeditions of the age, it claimed many lives, rested on the unsung heroism of its native members and only later would it be taken up as a blueprint for colonisation. Garnier, writer of the official journal and main survivor, would not scruple to scoop up the medals and plaudits for himself. Yet Keay generalises too widely in contrasting the official and diplomatic nature of this mission with those of the individualistic British. The latter drew on a wide spectrum of sponsorships, and there was an evolution from military to commercial, but they never entirely shed their governmental links. That Garnier would later perish in a crazed and monomaniacal attempted coup in Hanoi, without official sanction, shows the weakness of the contrast.

Nigel Barley is a writer and anthropologist and was formerly a curator at the British Museum.