In Plurimis §15

¹⁵ Toward the end of the fifteenth century, at which time the base stain of slavery having been nearly blotted out from among Christian nations, States were anxious to stand firmly in evangelical liberty, and also to increase their empire, this apostolic see took the greatest care that the evil germs of such depravity should nowhere revive. She therefore directed her provident vigilance to the newly discovered regions of Africa, Asia, and America; for a report had reached her that the leaders of those expeditions, Christians though they were, were wickedly snaking use of their arms and ingenuity for establishing and imposing slavery on these innocent nations. Indeed, since the crude nature of the soil which they had to overcome, nor less the wealth of metals which had to be extracted by digging, required very hard work, unjust and inhuman plans were entered into. For a certain traffic was begun, slaves being transported for that purpose from Ethiopia, which, at that time, under the name of La tratta dei Negri, too much occupied those colonies. An oppression of the indigenous inhabitants (who are collectively called Indians), much the same as slavery, followed with a like maltreatment.