Discovered this totally by accident. There seems to be a bug that lets you always win if your attacked (ground battles) without entering combat!
When the enemy attacks your city, you will get the option for the computer to automatically position your troops (OK Button) or not (CANCEL Button). Press the CANCEL Button and you go to the battlefield screen where you are given the opportunity to place your troops manually.
When you press the CANCEL Button the second time, the bug starts and somehow the next display indicates that you have successfully defended against the invaders. Especially handy when your playing the hardest level campaign.
This bug how ever has some "bugs" in it. It works most of the time, but it some occasions when you press the CANCEL Button the second time to start the bug, an error pops up (or in my case, Microsoft has detected an error..etc.), thus abruptly exiting the game. Even when you restore the game, the same bug "bug" problem happens on the same particular battle - kinda picky that bug. So save often in various slots!