Terra Incognita by Gary Hicks

Drawn together apparently by the hand of fate, Trent and Cole escape the oppressive totalitarian domed city that has nurtured them all their lives. Outside they discover a world they could never have imagined. They are pursued by the city Militia, intent on returning them to the secretive entity that oversees every facet of the city. They are joined by Floyd, who turns against the Militia; but then they are captured once more and once again freed by those they would have considered most unlikely to help them. The trio begin to understand the truth of their world and must decide for whom they will fight in the coming wars...