Get its war index down to 0. There is a pop up which tell you exactly how many points each target is worth and how many points you need to get to finally win. Please Nat, read some about how to do this, you MUST kill the enemies defensive interceptors and helicopters before attacking anything. If you dont, you will get slaughtered, spend way too much getting yourself into debt and probably get so cross you will want to quit.
So please make a plan first, its easy really, but if you have no idea, well....