[Suicide Junkie]

Have those Grome been building reinforcements by any chance?

In one (P&N) game I was playing, the planet I was attacking kept building more troops to fight off my invaders. Each turn, I killed all the troops and did only minor damage to the militia. Only once the planet started to riot was I able to win.
Took me two years.

I know I didn't mod that AI, aside from adding engines to ship designs, so either that was a coincidence, or the AIs are programmed to build troops on the planet being invaded.