I've read a fair bit on Julius Caesar, and Roman mid-late republic and early Empire, and I'm amazed by how comfortable people are with extremely harsh conquest and war. For example, it is said that Julius Caesar killed one million Gauls and enslaved another million, ( by his own account, I admit ) when the total population of the region was only ~8-16 million. He cut the hands off of all of those people in Uxellodunum and routinely burned entire cities to the ground, killing everyone he could just as an example. Even his most bitter foes in Rome had not one bad word to say about it. They exploded whenever Romans died however. To them, it was just the way things were. Yet more fascinating, Adrian's Goldsworthy's has a theory he discusses Pax Romana , the romans didn't need to justify brutal wars of conquest. considering Julius Caesar's specific actions in Gaul I can't help but agree. I am aware of the idea of roman 'pacification' and that breaking treaties with foreigners was heavily condemned, so they're not absolute. In the Middle ages, people used religion as a way of "othering" people, thus allowing brutal action against them, but even then this doesn't mean they can do whatever they want. Later on it became more focused on race. When the Europeans colonized Africa, most of them though they were helping them by 'civilizing' them and 'teaching them so they can one day be independent'. The Romans understood race and nations and religion, but they never stressed it enough to justify war, it was just a minor detail. The Romans, including Julius Caesar, seem to believe that 'history is written by the victors', 'we have a right to serve ourselves' and 'woe to the vanquished'. They don't go out of their way to be cruel, they are simply pragmatic and Machiavellian. If it benefits them they can treat the Gauls very well, enslave them, or kill them. There were no hard and fast rules of war, no moral obligation to the rest of the world, no absolute human rights, very few customs that foreign civilians as being a less appropriate target to kill than pedophiles and serial killers in Rome. What changed from then, and why doesn't that fly anymore? When did this mentality start and end? I'm sorry if I sound moralizing or critical of the Romans, I don't mean to. And of course, feel free to tell me if I've been misinformed.
I would disagree with strongly with Goldsworthy there, in fact Caesar's conquest of Gaul is one of the better examples of how the Romans did feel the need to construct justifications, both because we have the first have account of the person who led the conquest, and because of the historical circumstances around it. The second point first, the circumstance being that Caesar actually faced a credible threat of prosecution for the illegality of actions in his war in Gaul (in particular, the invasion into Germany) and this among other things was the inciting spark for his crossing the Rubicon. And we actually have evidence of debate in Rome itself at the time of his wars as to their justification in the form of Cicero's speech "De Provinciis Consularibus" which, among other things, is an extended justification for Caesar's actions in command. As for Caesar's words, On the War in Gaul constantly provides justification for each of his actions. Caesar never argues for his conquest from a purely utilitarian position either (ie, simply saying he thinks it would be useful to conquer this or that) rather he tends to argue he was defending either himself or his allies. This accords with what Cicero wrote in "De officiis", that Rome had gained its empire only fighting defensive, justified wars.
Now this is not to say that we need to take this claim seriously, or that the justification for conquest had to be particularly robust in modern terms, but they absolutely did require a justification of some sort.