Whether it's by jet, helicopter, or tank, your job is simple: protect the Earth from invading aliens that would like nothing better than to destroy all of mankind. Explore all kinds of terrain and undertake a variety of missions as you shoot first and ask questions later. If defending the planet is too much weight to bear, have a friend join you and see if teamwork can get the job done. Or you can play against a buddy and see who's more deserving of the title of Defender of the Earth.