Pulling the thread on slavery, I recently read a book on the history of Africa which revealed that African nations regularly conquered and captured members of neighboring tribes and sold them when they got a good offer. For hundreds of years, the export market was the Arabs. With the arrival of the Portuguese, the export market expanded to the Europeans taking the slaves to the New World.
It's worth noting that the American Indians were far more primitive than the Africans and therefore in many cases would simply kill their captives as they lacked the infrastructure to manage large numbers of slaves.
Moving onto the Europeans, I decided to read Julius Caesar's Commentaries. I was hoping to discover his attitudes towards the Germanic tribes. I did not. Instead, I found an expertly written military analysis of his campaigns in what are now France, Germany and England. While he doesn't bother with sophisticated descriptions of the locals, his analysis is telling.
Unburdened by Judeo-Christian notions that all men are created equal in the image of God, Caesar is the ultimate, secular Utilitarian. He has a job to do and he gets it done with minimal fuss. If you submit to Roman rule, even after battle, he takes hostages from your royal families and demands a tribute to pay for garrison legions. Your nation is allowed to survive, albeit as a vassal state to the Romans. If you betray that agreement or oppose him through any kind treachery, he defeats you in battle, burns your villages to the ground, slaughters your leaders and sells the population into slavery.
Lather, rinse, repeat.
There isn't a hint of remorse, regret or second thoughts and why should there be? There is no God, there is no objective morality, there are only the needs of the Roman people. It's a simple worldview and it works. It works really well.
Caesar has military superiority in organization, weapons, tactics and training. He uses it ruthlessly to expand the borders of the Republic to provide both a buffer zone for the safety of the Roman citizens and a stream of cash coming in from conquered nations. He makes his nation safer and wealthier. It's his job and he works constantly to improve his skills. His book is a lesson plan for future generations to study so they can conquer and enslave even more people.
What did he do wrong? Absolutely nothing as far as I can see.
Derive for me, from first principles, a moral argument against conquest and slavery in a world without God. If you can't come up with anything more efficient and effective than Caesar's solution, then it's hard to see how you don't end up there eventually.
Sort of like how the Chinese are dealing with their Muslim minority, no? The Chinese leaders are atheists whose culture has evolved over a long, long time and they've ended up with concentration camps. Why not? It seems to be working for them. Isn't that the measure of success in evolution?