The Mercenary

From the back cover Pocket Book paperback February 1977:

Why Has John Christian Falkenberg Thrown Millions of People on Dozens of Planets into Turmoil?

Because he's given up the brilliant military career he's known since age fifteen to fight on his own.

Because he's a legend to the Earth-bound, a mastermind to his soldiers, an enigma to the politicians who buy his services.

But mostly because he's John Christian Falkenberg.