2 Billion HP pirate cantinas

[fryman]

A cantina is a civilian building.

Civilian buildings cannot be destroyed while there is a military base on the world.

Is it possible this was the case? Was the pirate HQ building still standing?

Civilian cities/buildings CAN be damaged, which reduces their ability to manufacture things. Civilian cities can be bombed as a nuisance but they cannot be destroyed until the military base is removed. Then you could just build your own military base and all civilian buildings on the world become yours. You do not have to destroy civilian cities at all.

Yes, that was most likely the case. The HQ was still standing but at 0/750, and also was invulnerable at that point in that instance above screenshot. I was able to fix this by saving, exiting, reloading the game and giving the HQ another hit which was the one that finally destroyed it.