On a species level, you can imagine an aggression/cooperation "species personality" axis. Humans are in the middle, with chimps more on the aggressive side, and bonobos to the cooperation side.

Being in the middle, humans have a bit of a split personality. We cooperate on a large scale during warfare. But consider the Cold War. Both the US and the USSR were continent-spanning countries with multiple ethnicities. I would argue that cooperation on that scale just isn't that different from cooperation on a planet-wide scale. A species that's capable of one is very likely to be capable of the other. That's part of why I'm not terminally pessimistic about humanity, or starfaring species more generally.

I don't think we can rule out starfaring for a species that's a little more bonobo-like, and defaults to a post-national outlook.