What if your dad was deported about a year ago and you are about to become a us citizen when you join the army will you be able to bring him back to the us?

You need the services of an immigration attorney. If you have a spouse or children, they can be eligible for expedited citizenship, but it doesn't say anything about one's parents.

By the way, you don't instantly become a citizen by joining the military. You have to apply for it on your own, you must complete at least a year of service, and you need to apply while you're still in or within six months of your separation. Your chain of command should be able to get the ball rolling for you on that - I obtained expedited citizenship through the military, and it really does make the process a lot more convenient.