The Wizard and the War Machine by Lawrence Watt-Evans
At the end of "The Cyborg and the Sorcerers," Sam Turner was making a life for himself on the planet Dest. He thought he had left the long-lost interstellar war between Earth and its rebellious colonies behind him forever.
"Forever" turned out to be eleven years. That was how long it took for another Independent Reconnaissance Unit to respond to the distress call his ship had sent before it was destroyed.
And this one made his own berserk killer computer look sane.