Wookiee's versus the Na'Vi...
The Wookiee's versus the Na'Vi. Two scenarios:
Scenario one: Kashyyk has become uninhabitable. Global warming, whatever, the Wookiee race is forced to find a new world to make their own. They discover the moon of Pandora and decide that it is the perfect place for them to settle anew. The Na'Vi aren't having it, they will fight to defend their homeworld, so the Wookiee's decide to take the moon by force.
Scenario two: Same as one, except that the Na'Vi, after Pandora is destroyed by the sky people (Let's pretend that the Marines are successful in bombing the tree of souls), decide to resettle on Kashyyk. The Wookiee's view the Na'Vi as a threat and will defend their planet.
The Wookiee's have no orbital craft in either scenario. The Na'Vi can breath the air of Kashyyk, and the Wookie's can breath the air of Pandora. The armies are equal in number.
The following link is EU, but let's use it to make the thread more entertaining: http://www.starwars.com/databank/location/kashyyyk/ It explains the planet. The planet has several horizontal levels of ecology throughout the forests, with each lower level increasing in hostile danger.