Some were, some weren’t. It’s a conflict that goes back to the colonial era. Most of the soldiers fighting really had no use for planter society and owned no slaves. But, State was more akin to Nation in that era, and honor dictated that you answer when your people call. Doesn’t make sense to very many now, but that’s how it was.
You're right. Just as things would be if, hundreds years from now, there is a one-world government. Folks at that time would have trouble understanding why anyone would volunteer for a national army.