Lately I've become interested in the intellectual hoops that early modern Europeans were jumping through to justify their colonial expansion. Specifically I'm thinking about how rationalism and the scientific method were central to theories of racial essentialism and, by extension, racism in the modern sense. (I'm mostly getting these ideas from having read "A Genealogy of Modern Racism," so I guess my first implicit question is whether most historians accept that Dr. West was onto something...)
Based purely on depictions in popular culture, it seems like ancient Egyptians or classical Romans or medieval Scandinavians would have felt no need to justify conquest or enslavement of people beyond "because we can." Is this accurate? Or is there any literature suggesting that Romans or Vikings felt some need to defend the practice of slavery?
The Roman Republic, according to Livy, had an elaborate religious procedure for declaring war; this presumably ensured that the gods, and also the people, knew that the war was just. It is not clear whether this was actually a thing, nor how long it was practiced; but what seems clear is that Livy believed that Republican Romans had felt a need to justify their wars.
The Romans certainly seem to have had a sense that wars must be justified often using justifications that sound similar to more modern day nations. Caesar's De Bello Gallico has been seen as mostly about Caesar trying to justify his ever expanding conquests in Gaul (although there is certainly a large amount of propaganda of Caesars greatness too) to the Roman people and Senate, explaining his attack on the Helvetii as they were threatening Roman allies. There are examples of conquests to that were never adequately explained and the reaction of writers at the time seems to suggest this was abnormal, Domitian's war against the Chatti for example is criticised by Suetonius (or maybe its Dio? or both?) as being just for Domitian's Glory rather than any real security reason. Claudius' conquest of Britain has been treated similarly.
Citing prior post on the argument between Las Casas and Sepulveda in 1550.
Prior to [the encomienda system], the natives were sold outright into slavery, until such practice was banned under Isabella and Ferdinand. One argument was that native Americans had not previously been exposed to Christianity, so therefore they deserved instruction so that they would accept Christianity. The encomienda system was put in place with this in mind, and therefore in theory had very strong alignment with Spain's religious character. Of course, we now know that it was nowhere near the peaceful and mutually beneficial system that it sounded like in theory! When Las Casas first took up his cause in Spain in the 1510s, it took a very long time for his specific ideas to be formed, and for them to gain traction. So, one may ask, why did it take decades for the ideas to be accepted? Unsurprisingly, it had to do with politics. Las Casas' first mentor was Cardinal Ximenes/Jimenes Cisneros, confessor of Isabella and later regent of Castile. Cisneros thought Las Casas was too radical, even if he was in agreement with the Humanist views. When Cisneros passed away, Las Casas was able to gain the support of the Flemish party in Charles V's court. He knew that he needed a proposal of how to reform or outright replace the encomienda system, many of which were poorly implemented and failed miserably. Among other things, he advocated settlement of Castile's peasants and importation of African slaves, which I won't get into too much detail here. Further, over time his Flemish supporters either passed away or were caught in the Revolt of the Comuneros.
One primary opponent of Las Casas was Sépulveda, a strict Aristotelian, who was a champion of the idea of "natural slavery," justified by the inferiority of the native Americans (and other groups of people). He was not just a scholar, he was Philip II's tutor before Philip came of age. The arguments came to a head in a famous debate between the two in Valladolid in 1550, which itself is testament to the vigorous intellectual tradition at the time even if it had taken Las Casas decades to secure that opportunity.