The slave trade began with Portuguese (and some Spanish) traders, taking mainly enslaved West African (and some Central African) people to the American colonies they had conquered in the 15th century. British sailors became involved in the trade in the 16th century and their involvement increased in the 18th century when the Treaty of Utrecht (1713) gave them the right to sell enslaved people in the Spanish Empire. The slave trade made a great deal of profit for those who sold and exchanged enslaved people. Therefore, they often ignored the fact it was inhuman and unfair.
Between 1532 and 1832, at least 12 million African people were enslaved and taken to the Americas, and at least a third of them were taken in British ships.
For the British enslavers it was a three-legged journey called the 'triangular trade':