Watch the military briefing. It was the Humans making a desperation attack, trying to break the spirit of the Natives with what amounted to a hit on the exhaust port. They rushed into an arena that negated most of their technical advantages and still almost succeeded until they were defeated by the very thing their scientists had been insisting they didn't fully understand. Militaril

It's not all that farfetched, minus the whole "it's an alien world" thing. You are correct, however, in that if the Humans return in force, the Pandorians will need something else on their side. Of course, as was explained early on, this was a small band of what amounted to mercenaries. Could be the Company has been bankrupted. But it's a Cameron movie, so we will see a sequel.