I really liked Unity of Command. Operational-level, Eastern Front, WW2. Available for Linux.
The AI is excellent especially when playing the defense (which is nice, since for the human playing the attacker is the more interesting role). It's great at noticing when the player has overreached and mercilessly punishing those mistakes.
I can't recommend it highly enough if you have any interest at all in wargames. It works as an entry level game, but is also deeply satisfying for the grognards.