Novel by Jerry Pournelle from 1977. Later included in Falkenberg's Legion (1990) and in The Prince (2002). In the latter it's divided in two parts, Hadley and New Washington, named after two of the planets on which it takes place. The latter was originally a short story called Sword and Scepter? before it was included in The Mercenary.
Sooner or later, all civilizations age and decline, grow senile and die—either by conquest from without, or from internal rot. The final days of Western Civilization are signaled by the joining of the USA and the USSR into a ruthless and imperial state that spans first the Earth and then the Solar System. But, because it fails to hold the loyalty of its soldiers, it falters at the stars.
The Mercenary is John Christian Falkenberg. He's a legend to the Earth-bound, a mastermind to his soldiers, an enigma to the politicians who buy his services. He's given up the brilliant military career he's known since age fifteen to fight on his own.