Revenant Gun

"Shuos Jedao is awake... and nothing is as he remembers. He's a teenager, a cadet--a nobody--in the body of an old man; a general in command of an elite force. And he's the most feared, and reviled, man in the galaxy. Jedao carries orders from Hexarch Nirai Kujen to re-conquer the fractured pieces of the hexarchate. But he has no memory of ever being a soldier, let alone a general, and the Kel soldiers under his command hate him for a massacre he can't remember committing. Kujen's friendliness can't hide the fact that he's a tyrant. And what's worse, Jedao and Kujen are being hunted--by an enemy who knows more about Jedao than he does himself..."--

Synopsis from Open Library.