From a Russ Roberts essay back in 2003:
….The economists who favored abolishing the draft had to answer the charge that there was something unseemly about paying soldiers enough to step forward. During the hearings on abolishing the draft, Gen. William Westmoreland challenged Friedman and asked him how he felt about being defended by an army of mercenaries. Friedman’s answer was that he preferred mercenaries to slaves.
Besides losing the rhetorical debate, Westmoreland misunderstood the role of pay in motivating workers. Doctors earn a nice living in America but that doesn’t rule out compassion and care as part of the motivation for going to medical school. Paying people to serve in the military doesn’t rule out people stepping forward who love their country and want to serve.
Thirty years ago, the volunteer army was a great experiment. Today, even the military has embraced it. The quality of recruits is higher than it was, and morale is strong. And the American people seem to like it, too. In a poll last year for ABC News and the Washington Post, 97 percent of the American people said they were either very proud or somewhat proud of the armed forces. Pretty good for mercenaries.