The Highest Treason by Randall Garrett, Science Fiction, Adventure

TRAITOR TO THE HUMAN RACE It was a space war on a massive scale against an alien enemy sworn to enslave mankind on Earth and all its colonies . . . But mankind's society had grown weak, forgetting the key component that had made it reach for the stars in the first place. Now it was mired in a bland, homogenized, politically-correct mess, bankrupt of values. Sebastian MacMaine, a master strategist, abandoned the Earth Empire to work for the Kerothi and their alien space fleets. His was the most startling mission in human history. His tactics? Ruthless.