I'm not sure about the 20th century, but I do know that it was fairly normal in Italy during the Renaissance to hire private armies - called Free Companies - to do your fighting for you.

Federico da Montefeltro was a condottiere, i.e. a leader of such a Free Company, as well as being the Duke of Urbino. His private army made him one of Italy's wealthiest men and, aside from being a great military leader, he was also a great patron of the arts and letters, supporting artists like Piero della Francesca and Raffaello, and building one of the greatest libraries in all of Italy.

One that comes to mind is Lt Col John Paul Vann (US Army).

Lt Col Vann served in the US Military, including service in Vietnam, until 1963.

In 1965 he returned to Vietnam as a Civilian Advisor. By the early 1970's he was, as a civilian, in a position where he was commanding both ARVN and US Military forces at a level equivalent to a Major General; the first civilian to command US Regular forces in combat.

See "A Bright Shining Lie: John Paul Vann and America in Vietnam" by Neil Sheehan and also http://en.wikipedia.org/wiki/John_Paul_Vann#Civilian_career .