https://www.beyondallreason.info/

Looking at it, the engine is GPL: https://beyond-all-reason.github.io/spring/guides/choose-recoil

But while BAR releases their source, it isn't really open source (no derivatives): for example, with the many of the art assets, "all rights reserved": https://github.com/beyond-all-reason/Beyond-All-Reason/blob/master/license_general.txt

It does look good, that said.

The Recoil engine is interesting to me. It's not something I'd seen before in the open source ecosystem (sometimes I live under a rock). I suspect the underlying mechanisms (although written for RTS games in the Total Annihilation family) might permit implementing something like a Paradox game with some tweaking.

I have played countless hours of this. Even against AI, this game is great fun. It's mind-blowing to think it's free.

It's a fun game. Brings me back to playing total annihilation as a kid. Chaos everywhere.