Slavery was a socially accepted and promoted practice in Greek and Roman antiquity and in Eastern empires. The modern practice of human trafficking is another iteration in the history of the international slave trade. An intricate global network connected traders in Africa to merchants in emerging European countries, as well as buyers in the Americas. Warring states enslaved surviving opponents and kidnapped people from other states to provide bodies for slavery. Trade expanded when Portugal and other European powers built large fleets capable of sailing to Africa and trading with merchants involved in the Arab and African slave trade. Spanish explorers imported West Africans to the Americas, establishing a practice that lasted for over three centuries. Historians estimate that 11 million slaves were transported from Africa to the Americas, and over one million died from violence, smallpox and other fatal diseases during the tumultuous journey across the Atlantic known as the Middle Passage. An estimated total of 25 million Africans were removed from the continent and sold into slavery. Many African kingdoms also practiced local forms of servitude, and self-interested African merchants and rulers sold humans to European merchants before demand from slave labor reached the point that European powers conducted slave raids. Colonialism buttressed by a racial ideology that touted the superiority of Europeans to Africans and other non-white races grew concomitantly with the burgeoning slave trade. Empires and kingdoms required free labor to expand and maintain their global presence, and improved maritime and navigational skills buttressed their expansion. The historical human mass displacement has resulted in global diasporas serving as remnants of a once powerful institution.