Maybem, though I'm not saying you need to turn the planet into a crater, but it's way safer to take out the hometree with a few missiles from space then some massive frontal assault. Either way, the unobtanium is just a rock, and it has to be mined out anyways, so I'm assuming it would be OK as it's in the ground already. And if not, there's always chemical weapons or napalm. It just seems very nonsensical that their only option was to try to fight on such skewed terms like that.