The expansion of Muslim power through the Ottoman conquest of large parts of the eastern Mediterranean in the 15th and 16th century resulted in extensive piracy on sea trading. The so-called Barbary pirates began to operate out of North African ports in Algiers, Tunis, Tripoli, Morocco around 1500, preying primarily on the shipping of Christian powers, including massive slave raids at sea as well as on land. The Barbary pirates were nominally under Ottoman suzerainty, but had considerable independence to prey on the enemies of Islam. The Muslim corsairs were technically often privateers with support from legitimate, though highly belligerent, states. They considered themselves as holy Muslim warriors, or ghazis, carrying on the tradition of fighting the incursion of Western Christians that had begun with the First Crusade late in the 11th century.