It'll use its aircraft on your mobile artillery and perform some sort of counter-battery fire if it can. Sometimes it seems as if too much emphasis is put on the frontline, so when you do break though the computer's defences there's little held in reserve to halt your advance. Almost certainly not true, but if you take to the game system used there is quite possibly a few weeks worth of action before you've fully explored all the scenario styles. You can pick short small scenarios or large campaign games - the four campaigns link between 3 and 9 battles, where to progress to the next battle you need to achieve a goal in the current one. This may involve getting X amount of points worth of units to a given area by the scenario's end, or possibly just destroying X points worth of the enemy's units.
Scenarios vary in balance too, some do put one side up against it through sheer odds of numbers.
An interesting feature here is you can play a scenario from both sides and be awarded an overall score - a bit like in bridge where both teams get to play both sets of cards. When you complete a scenario your game is recorded in a huge statistical database, so you have highscores for each scenario, at each level, for all players. Until I sorted out my mouse driver, I could only autoscroll in two of the four possible directions, which was a pain, but I have no problem now. TPG II is quite bug free. Certainly in many hours of play it's not crashed on me, and I have a fairly standard setup.