By the time America won its independence in 1783 upwards of 50,000 British criminals had been banished to American shores. The Americans wanted no more and, with the old route stopped up and prisoners piling up in British jails, London had to find another dump. In 1788 the First Fleet to Australia entered Sydney Cove and a new penal colony was born. Between those dates, however, reckless, misconceived and tragic attempts were made to ship convicts off to the slave forts of West Africa.

A Merciless Place is the story of these West African experiments and could have been merely an expanded footnote to two familiar areas of 18th century study: the European settlement (or ‘invasion’, as it is now officially known) of Australia and British slave trading in Africa. On the one hand it is the sorry tale of an African penal colony that failed, just before the slightly less sorry Australian endeavour worked (at least from some points of view). On the other it is a snapshot of the tangled, multiple relationships between slavers and slaves, whites and blacks, Europeans and Africans at the height of the ‘vile traffic’. But Christopher’s book is more than the sum of those parts. It is also one of the increasingly rare works in this age of televisual big hitters to examine not the powerful individuals who determined international relations with pen and sword, but those who bore the consequences of their actions and decisions: the little people at the bottom of the heap.

Christopher’s tale starts and finishes in familiar territory: London at the start of the 1780s and Sydney at the end of the same decade. It is set during the epic generational battle of the 1770s and 1780s between mother Britain, adolescent America and the newborn terra nullius of Australia. These family struggles determined the fate of countless people who knew nothing of the international buzzwords of the moment – liberty, tyranny, democracy, slavery. People sentenced to transportation in the same London court might end up in America, Australia or Africa, or remain in Britain, according to circumstance and accident, strength of personality and political whim. Among them were the hapless individuals of Emma Christopher’s tale.

The first section, in which Christopher introduces her principal characters, is set in the grimy, corrupt Britain of George III and couched in a familiar idiom of disgust. So much has been written about 18th-century crime, particularly since the Old Bailey records went online, that the details are in danger of becoming stale, but Christopher’s cast of characters is nonetheless fascinating: among them wily thief Patrick Madan, worth a biography in himself; fraudster William Murray, already transported to and illegally returned from America, who met one of the nastiest ends imaginable; and his killer, the sadistic army officer Kenneth McKenzie, who committed a catalogue of atrocities.

Christopher then focuses on two overlapping African expeditions. The first, in 1781, saw capitally condemned men ‘pardoned’ on condition of enlisting as soldiers under McKenzie and sailing to guard British slaving forts and to attack the more successful Dutch ones. Thus unfolds an astonishing tale of madness, conspiracy and cruelty in the ‘dying grounds’ of West Africa, where malaria killed many within weeks of landing. The second, in 1783, saw ‘civilian’ convicts (including two women) sent, with an unbelievable lack of planning, to establish a settlement on an island in the River Gambia. The nearest British governor, already without resources to feed and defend his own people and premises, told them to fend for themselves. None survived.

In part because Christopher is principally a historian of the slave trade – her last book was about the sailors who manned the slave ships – the African section of her book is the longest and most interesting and her knowledge of British interventions and personnel on the slaving coasts add enormously to her account. Particularly telling are the reactions of white slavers to the prospect of undermining fragile racial hierarchies, if white convicts were seen being treated as slaves.

After the suffering of the African convicts it is almost a relief to turn, at the end of the book, to the First Fleet under the care of sane, humane Arthur Philip; and a grim irony to read of Kenneth McKenzie’s eventual pardon for crimes far worse than those committed by the convicts under his African command.

Siân Rees is the author of Moll: The Life and Times of Moll Flanders (Chatto & Windus, 2011).