Complex sentence: Until the late 18th century, Crimean Tatars maintained a massive slave trade with the Ottoman Empire and the Middle East, exporting about 2 million slaves from Russia and Ukraine over the period 1500--1700.
Split into simpler sentences: Until the late 18th century, Slavery in the Ottoman Empire thrived, with Crimean Tatars selling slaves to the Ottoman Empire and the Middle East. About 2 million slaves from Russia and Ukraine were sold over the period 1500--1700.