Actually, it's not. Put a nuclear bomb in a lead tube, or just in water and it's completely invisible. It won't last, but it'll work.
I'm not so sure. The Kai are the more advanced faction here. They can develop ECM, and all sorts of antimissile technology. Also, still no answer about being targeted by rocket artillery, or conventional artillery.
The Vietnamese didn't have fusion powered warships, and didn't have complete control of an entire planet.
Kai can dig. And have perfectly selfsufficient lifesupport systems. Also, underground base was kinda an understatement. Our original base was, I dunno, 400 meters deep*. Had multiple selfsufficient lifesupport systems, blast doors, and there were plans for even deeper, larger bases. Things designed to withstand orbital bombardment on a massive scale...
Also, the Kai have bigger guns than you. That's kind of a certainty. They're the more advanced factions, and have plenty of time to reverse engineer everything.
*There were surface levels for the flowers.
The US didn't have walking tanks, and I have the audacity to be certain that if they throw an army at me I can grind it into dust. Probably suicidal I know but hey, audacity has served me well so far and on the last eventuality like that in a game I played I took down an army of 29000 men with 250 of my own. Luck has a way of favoring the bold with dice rolls.
Those odds are a major victory for the Kai, you know. Exponential growth and that kind of stuff.