The Atlantic slave trade is more widely evoked, depicted and debated in our culture than any other historical crime or tragedy except the Holocaust. There are good reasons for this. The New World slave systems fed by the Atlantic trade were different from all others, including those within Africa itself, because they alone formed a central, dynamic element in the growth of the modern world. Their sheer scale was unique: at least 12 million Africans were transported or died in the process, perhaps many more. The racial ideologies that sanctioned them still taint every society involved. Some historians argue that Britain would never have had its industrial revolution, nor the United States risen to global dominance, without the profits from slavery.
Yet the Atlantic trade was not the only one. The civilisations of south and south-west Asia held and traded slaves on almost the same scale as Europe. Estimates of the numbers enslaved in the trans-Saharan, Red Sea and Indian Ocean commerces vary enormously. They went on for much longer than the Atlantic slave plunder, and far fewer written records survive or have been found. Totals, across the centuries, of anything from three to 14 million have been suggested, with the most detailed calculation proposing just over seven million. The predators involved, and the societies to which these slaves were sent, were mostly Muslim. The whole complex is thus conventionally called "the Islamic slave trade" - a practice that Ronald Segal follows. But this is just as dubious as calling the Atlantic slave trade "Christian".
It was very different in character from the European-dominated enslaving business. Slaves were employed more in domestic service and as soldiers, far less as agricultural labourers, than in the New World. It involved far more enslaved women - whether as servants, concubines, or both. Some have argued that "Islamic" slavery was in the main more humane than the European form. It was easier to buy or be granted one's freedom; ex-slaves or even people who were still, on paper, enslaved could rise to high office. Certainly, in the Islamic world, there were few equivalents of the vast, brutally exploitative New World plantation systems - nor, apparently, equivalents of the hideous mortality rates that many of these exacted. In the longer term, it is evident that the social assimilation of former slaves and their descendants, including intermarriage, has been far more the norm in Islamic societies than in the west. They had, and still have, comparatively little institutional racism, to which Islamic teaching is resolutely hostile.
But the balance of moral judgement is fiercely contested. Enslavement of Africans in Islamic societies not only started earlier than in Christian ones, but continued later. The widespread castration of male slaves was as cruel as anything practised by Americans. And it was European Christians, not Africans, Asians or Muslims, who eventually abolished the institution of slavery and forced others to do so - in the late 19th century, the drive to end the "Arab slave trade" was offered as a major justification for European colonial expansion in eastern and central Africa, even if there was much hypocrisy in such claims.
Segal, a veteran South African radical, offers an effective, accessible synthesis of existing writing about the slavery of the Islamic world - which, compared to American slavery, has been much neglected. His book does not say much that is new, but it is immensely useful to have around. It is the latest, and one of the best, of the many broad-sweep over-views of big subjects that Segal has been producing for 40 years, and it is certainly more successful than its precursor, the superficial Black Diaspora (1995). It reminds us, too, that this is not just a historical issue: in several African and Middle Eastern countries, notably Mauritania and Sudan, slavery still exists in all but name.
Stephen Howe's most recent book is Ireland and Empire (Oxford University Press, £12.99)