They need water for fuel or something. So they come here cuz it's the only place in the universe where it's in liquid form (wrong). Also, they somehow bury a control command center deep within the center of the city (not explained). This lets them fly unmanned drones. Also, the humans discover they are basically invulnerable, except for one tiny spot, which they then manage to somehow hit every time for the rest of the film. They get stopped by blowing up the top of the command center, and then drones lift the command center out of the ground to make it easier for the Marines to blow up the rest of it.

The end.