Portugal began trading enslaved Africans in the 1440s, over 200 years before Britain became a significant participant. For the first century and a half of the Atlantic trade, Portugal had a near-monopoly enforced by the Treaty of Tordesillas. The infrastructure of West African trading forts, the commercial relationships with African kingdoms, and the plantation systems in Brazil were all established by Portugal before other European nations entered the trade in any serious way.
Portugal’s dominance meant that it bore primary responsibility for establishing the systems that made the trade possible at scale. Britain inherited and expanded structures that Portugal built. The moral weight of founding the trade and maintaining it for four centuries is at least as great as the moral weight of participating actively in it for 150 years.
