Shadow Warrior Trilogy #1: Wind After Time by Chris Bunch

BLAST FROM THE PAST
Ten years had passed since mankind had won the Great War. The dreaded Al’ar, loathsome aliens who killed by mere sight or touch, had been vanquished. Now they were gone, forgotten, and humanity was free to expand throughout the galaxy.
But one man remembered. Joshua Wolfe had been a hero of the Great War. He had lived with the Al’ar first as a friend and then as a prisoner. He knew their ways, knew their disciplines, knew how to kill as they killed.
And Joshua Wolfe was the only man who knew that the conflict was not over.
Before the Great War, in a time of friendship, the Al’ar had given him a name: Shadow Warrior. Now he would fulfill its lethal promise…