The central theme is in the game's name- Destiny - just a single word removed from 'manifest destiny.' It becomes apparent that the taking of these worlds are our manifest destiny. This isn't the case merely because "it's a videogame, just accept it;" it's because these worlds were once ours. They were taken from us. And they must be ours once again. Destiny works because the ruins of The Cosmodrome, The Moon, Venus and Mars are a constant reminder of what was taken from us. And it's the central motivation that drives Destiny forward.