[Warning: spoiler-heavy]<p>I have to say the exact same thing occurred to me at the end of the movie. "And so the aliens returned to their dying world." ...yeah, to get more ammo.<p>Of course, the counter-argument might be that the Na'vi managed to defeat a heavily-armed base using a skeleton force, before most of the tribes had even turned up, so by the time they got back Pandora could be too well-prepared to resist for them to be successful. Kinetic bombardment is all very well, but you need to get down to the surface to pick the stuff up -- unless you completely destroy every living thing on the planet, in which case you develop a very serious logistical problem (they were growing their own food on the surface).