The Variable Man — Philip K. Dick

In The Variable Man, Earth (The Terran System) is in a cold war with the older, corrupt Centaurian System, suppresses Terra from exploring space and expandig its borders. When a devastating new weapon called Icarus is developed by Terra, everyone expects the odds to tip in Terra's favor. But the computers calculating these odds find an unknown variable that throws off the odds. A time traveler from 1913, known as Thomas Cole has arrived. Can this variable man from the past rescue mankind from a catastrophic future?