The Portuguese were the first to engage in the Atlantic slave trade in the 16th century. In 1526, they completed the first transatlantic slave voyage to Brazil, and other European countries soon followed. Shipowners regarded the slaves as cargo to be transported to the Americas as quickly and cheaply as possible, there to be sold to work on coffee, tobacco, cocoa, sugar and cotton plantations, gold and silver mines, rice fields, construction industry, cutting timber for ships, in skilled labour, and as domestic servants. The first Africans imported to the English colonies were classified as "indentured servants", like workers coming from England, and also as "apprentices for life". By the middle of the 17th century, slavery had hardened as a racial caste, with the slaves and their offspring being legally the property of their owners, and children born to slave mothers were also slaves. As property, the people were considered merchandise or units of labour, and were sold at markets with other goods and services.
The major Atlantic slave trading nations, ordered by trade volume, were: the Portuguese, the British, the French, the Spanish, and the Dutch Empires. Several had established outposts on the African coast where they purchased slaves from local African leaders. These slaves were managed by a factor who was established on or near the coast to expedite the shipping of slaves to the New World. Slaves were kept in a factory while awaiting shipment. Current estimates are that about 12 million Africans were shipped across the Atlantic, although the number purchased by the traders was considerably higher, as the passage had a high death rate. Near the beginning of the 19th century, various governments acted to ban the trade, although illegal smuggling still occurred. In the early 21st century, several governments issued apologies for the transatlantic slave trade.
The Atlantic slave trade developed after trade contacts were established between the "Old World" (Afro-Eurasia) and the "New World" (the Americas). For centuries, tidal currents had made ocean travel particularly difficult and risky for the ships that were then available, and as such there had been very little, if any, maritime contact between the peoples living in these continents. In the 15th century, however, new European developments in seafaring technologies resulted in ships being better equipped to deal with the tidal currents, and could begin traversing the Atlantic Ocean. Between 1600 and 1800, approximately 300,000 sailors engaged in the slave trade visited West Africa. In doing so, they came into contact with societies living along the west African coast and in the Americas which they had never previously encountered. Historian Pierre Chaunu termed the consequences of European navigation "disenclavement", with it marking an end of isolation for some societies and an increase in inter-societal contact for most others.
Historian John Thornton noted, "A number of technical and geographical factors combined to make Europeans the most likely people to explore the Atlantic and develop its commerce". He identified these as being the drive to find new and profitable commercial opportunities outside Europe as well as the desire to create an alternative trade network to that controlled by the Muslim Empire of the Middle East, which was viewed as a commercial, political and religious threat to European Christendom. In particular, European traders wanted to trade for gold, which could be found in western Africa, and also to find a maritime route to "the Indies" (India), where they could trade for luxury goods such as spices without having to obtain these items from Middle Eastern Islamic traders.
Although many of the initial Atlantic naval explorations were led by Iberians, members of many European nationalities were involved, including sailors from Portugal, Spain, the Italian kingdoms, England, France and the Netherlands. This diversity led Thornton to describe the initial "exploration of the Atlantic" as "a truly international exercise, even if many of the dramatic discoveries were made under the sponsorship of the Iberian monarchs". That leadership later gave rise to the myth that "the Iberians were the sole leaders of the exploration".