This investigation compares the Spanish cleric, Bartolomé de Las Casas, with the Portuguese cleric, Fernando Oliveira, both of whom raised their voices in protest during the sixteenth century as Portugal and Spain politically extended, economically exploited, and religiously expanded into the Atlantic World. Las Casas condemned the wars of conquest and the consequent unjust enslavement of New World indigenous peoples; Oliveira condemned unjust warfare waged to promote and sustain the slave trade along the West African coast. This analysis demonstrates that both priests denounced warfare that violated the principles of just war and therefore resulted in enslavements without just cause.