Battle Royale question

(THE BOOK BATTLE ROYAL ISN'T THE BOOK BUT IT WAS NEEDED TO POST) So I read this book like 4-6 years ago, back then I think it was newer. I Don't remember much about it, but what I do remember is that it was a modern/near future civil war in the usa, the back-story is you had a religious cult kind of militia that had taken (at least I believe) half the country separated from Dakota or something like that. The story centers around a boy that was forced to join militia (because if he didn't they would give a choice join there cult/militia or die) and he was a very low rank in the militia and I believe in the beginning it started with him in the desert and he had a dislocated/broken arm/shoulder and was picked up by a force off US soilder, and he gains there trust and tricks them, to where there base gets taken by the militia, later on he is supposed to capture a dog for training, but he didn't want to because they would make it mean and blood thirsty for there cause, so he decided to let it go and just say it escaped, but then a higher rank or offixer I think sees him, and tells him to bring it back he kills him (I don't remember if it was an accident). Then he runs back to the base where his younger brother is, and grabs him and they run away, but later on his brother says he wants to stay because he worked way to hard to just leave and because he believed in the cult full heartedly. After that I forget other than he went to Dakota or something and meets a rich family (which is why I believe the civil war happened is because of economic eqaulity) and I remember reading that he couldn't help to think that these guy were swimming in a pool/lake while there was a war going on.
That's about all, please help me, I really need to find this book.