> it was a time when all major powers used slaves

This is really key here. There's a trend to hold people of the past up to standards of the present, but the world was extremely different then. Before the advent of farming machines and nitrogen fixation, labor dictated the power of a nation, labor relations and unions weren't a big thing, and a huge majority of the population worked in agriculture. Because there were no nuclear weapons capable of destroying the entire planet, superpowers commonly fought for resources, meaning the surviving nations had to be strong. The world is different.