October Novel Prep Challenge

I need some help brainstorming. I'm a little dead in the brain right now from work (Starbucks will do that to you), so I can't even think around this problem. It's a background story that sets up the plot. Problem is that half of the book is the background story, you know? The main plot is the bit with my main character and whatnot (as it should be), with bringing down the evil government. The secondary plot is the background of that evil government--explaining how it came to be--and features my main antagonist...well, the closest thing to a human antagonist I've got. So, I'm not sure how to write a background story without actually writing part of the novel and I obviously don't want to cheat and write some of the thing in advance. I'd thought about writing a background story to how the Coalition came to be (the terrorist/freedom fighter group), but I'm not sure...
-Quaddy