mnaintaiing two focuses at once. You have thos huge war where the enemy's goal is purging of those who corrupt the 'sacred planet' and the the enslavement of the rest of the people who inhait the other Colonial Federation world (Earth mostly uninhabitable but it is also essentially the new jerusalem. The plot goes deeper than that though. Much deeper). At the same time it also focses o this suad of Marines fighting this evil enemy. The story is about fighting for freedom at its core.
That sounds like a nicely layered plot.