In the movie 'Avatar' why did the marines want to destroy the trees?

The RDA (Resources Development Administration) is on Pandora to mine Unobtainium. Research showed that the largest deposit of this element in the area is located under the Na'vi Hometree. By knocking the tree down they would be able to mine. The soldiers went after the spiritual trees such as the Tree of Souls to break the spirit of the Na'vi people.