Avatar 2; this is actually here as a placeholder to link to from other places

"Avatar 2?  Sounds like a pretty short movie. They come back and nuke the giant smurfs from orbit. Game over, man."

Not really. The earliest Earth could get the news and have a fleet return is 13 years. After the initial landing of security forces, the remaining ships are supplies and personnel, and are set up ona rotating basis. Essentially a ship spends 6 3/4 years in transit, one year in layover (on Earth/Pandora) and then goes back. It'll be nearly 7 years before they even notice one of their ships comes back off-schedule, and they'll still have dispatched all the return trips with nothing but supplies and smallarms.

The Na'vi and their human sympathizers are presently in control of a MASSIVE amount of hardware. After the defeat at hometree, other bases simply surrendered and were sent home on the ship. The Na'vi will then simply waylay the incoming ships, take the intended supplies, and send them back emptyhanded. Until that first one arrives on Earth to discuss their terms of trade. The supplementary material makes this clearer, but Avatar's Earth would go dark in 20 years without Unobtanium; finding Pandora in the first place literally saved Earth's ass.

At this point the humans have to decide "Is it worth what little resources we have left to mobilize a fleet against an entrenched people that have undoubtedly assimilated and (given all the scientists that stayed behind) likely improved on our tech by now, or will we play nice and trade for Unobtanium?" As it stands currently, Earth is STRIPPED of resources, and Unobtanium is about the only hope the human race has.

War with an uncertain outcome, or a trade agreement of some sort? My guess is the next movie in the series will be about 50 years after the first, mired in some events once Mother Earth gets back on her feet enough to decide if they want to attack or not.