> Alternatively, advanced megaprojects are only achievable through sophisticated large-scale cooperation.

In that sense, war is a megaproject. War organized the Manhattan Project, which is still the metaphor we use for any massive scale, sophisticated project. The space race was a cold war endeavor to make ICBMs that weren't obviously ICBMs, and the Soviets were terrified that the Space Shuttle was a nuclear dive-bomber (actually it was for deploying and returning recon satellites) [0]. Cooperation does not necessarily imply peace or post-nationalism: war is strong cooperation on each side of the war, with competition between the two sides. In fact, the cooperation is so strong that actions taken against that cooperation end up being punished as treason much more strenuously than in peace time.

[0] https://www.thespacereview.com/article/3855/1

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.

The Space Race was partly a Cold War propaganda program but it diverged almost completely from the ICBM programs. ICBMs have to be solid fueled to minimize launch time. But manned orbital launchers have to be liquid fueled (for the core) for efficiency and safety.