Racism as we know it has only been a thing since the days of slave trade and colonization, probably as a way to rationalize it. The early day and medieval empires (Roman, Mongol, Inca, etc.) were very much not racist given how they had to federate many different populations and coerce them into living together.

It doesn't mean there weren't any in-group dynamics or discrimination though. Race simply wasn't the focus. People would discriminate based on things like religion instead.

What do you mean by racism? Discriminating against people based on their race? I think the reason we've forced ourselves not to be racist is because it's morally wrong.