“They are volunteers: last of the great volunteer armies, for the draft is beginning that summer in the North. They have lost faith in their leaders but not in themselves.”

I Ain't Got Time To Bleed (1999)

Referring to Allied Intervention in the Russian Civil War Tim Buck A Conscience for Canada

Source: The Hunger Games

Source: An Economist's Protest: Columns in Political Economy (1966), p. 121

And this is much more broadly based. In fact, I think mercenary motives are among the least unattractive that we have.
The Draft: A Handbook of Facts and Alternatives, Sol Tax, edit., chapter: “Why Not a Voluntary Army?” University of Chicago Press (1967) p. 366, based on the Conference Held at the University of Chicago, December 4-7, 1966

Anarchism: Its Philosophy and Ideal (1896)
Context: These societies already begin to encroach everywhere on the functions of the State, and strive to substitute free action of volunteers for that of a centralized State. In England we see arise insurance companies against theft; societies for coast defense, volunteer societies for land defense, which the State endeavors to get under its thumb, thereby making them instruments of domination, although their original aim was to do without the State. Were it not for Church and State, free societies would have already conquered the whole of the immense domain of education. And, in spite of all difficulties, they begin to invade this domain as well, and make their influence already felt.
And when we mark the progress already accomplished in that direction, in spite of and against the State, which tries by all means to maintain its supremacy of recent origin; when we see how voluntary societies invade everything and are only impeded in their development by the State, we are forced to recognize a powerful tendency, a latent force in modern society.

Roy Edward Disney (2003) as quoted in Disney Stories: Getting to Digital (2012) by Newton Lee and Krystina Madej, p. 4