If that's the kind of story that interests you, then check out Gratuitous Tank Battles
. It's a hybrid tower-defense/RTS/sim that allows you to play either attack or defense, fully customize units, edit and upload maps, save you own attack strategies so you can defend against them, and features adaptive AI.
Gratuitous Tank Battles
has entered its beta-testing phase today and is now available for pre-order straight from the developers
. Pre-order now and you'll receive access to the current beta version. If you buy it now, you'll get a Steam key for when the game is released onto Steam (it will also become available via other online portals).