After a revolt in the mid-17th century reduced the ruling Ottoman Pashas to little more than figureheads in the region, the towns of Tripoli, Algiers, Tunis and others became independent in all but name. Without a large central authority and its laws, the pirates themselves started to gain much influence. They justified the slave trade with Islam.
In 1785 when Thomas Jefferson and John Adams went to London to negotiate with Tripoli's envoy, Ambassador Sidi Haji Abdrahaman, they asked him what right he had to take slaves in this way. He replied that the "right" was "founded on the Laws of the Prophet, that it was written in their Koran that all nations who should not have answered their authority were sinners, that it was their right and duty to make war upon them wherever they could be found, and to make slaves of all they could take as prisoners, and that every Mussulman who should be slain in battle was sure to go to Paradise".
Of course, these estimates are low. The likely size of the trade in white people was likely much higher.
The power and influence of these pirates during this time was such that nations including the United States of America paid tribute in order to stave off their attacks. Supplies from the Black Sea appear to have been even larger. A compilation of partial statistics and patchy estimates indicates that a little fewer than 2 million Russians, Ukrainians, and Poles were seized from 1468 to 1694. Additionally, there were slaves from the Caucasus obtained by a mixture of raiding and trading. 16th- and 17th-century customs statistics suggest that Istanbul's slave import from the Black Sea may have totalled around 2.5 million from 1450 to 1700.
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