“We have now done what the Romans did when they started to commit suicide. We have shifted from an army of citizens to an army of mercenaries…”

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

Variant translation: Two armies that fight each other is like one large army that commits suicide.
Under Fire (1916), Ch. 1 - The Vision

That was the last that we heard from the general about mercenaries.
Two Lucky People: Memoirs, Chicago: University of Chicago Press (1998) p. 380.

And this is much more broadly based. In fact, I think mercenary motives are among the least unattractive that we have.
Source: The Draft: A Handbook of Facts and Alternatives, Sol Tax, edit., chapter: “Recruitment of Military Manpower Solely by Voluntary Means,” chairman: Aristide Zolberg, University of Chicago Press (1967) p. 366, based on the Conference Held at the University of Chicago, December 4-7, 1966, also in Two Lucky People, Milton and Rose Friedman, Chicago: University of Chicago Press, 1998, p. 380.

49
Multitude: War and Democracy in the Age of Empire

Brooks D. Simpson. "The Soldiers' Flag?" https://cwcrossroads.wordpress.com/2015/07/05/the-soldiers-flag/ (5 July 2015), Crossroads, WordPress
2010s

Referring to the importance of well trained militia amidst the populations of the states and their preferability to standing armies, in a letter to James Monroe (19 June 1813), published Thomas Jefferson to James Monroe, 19 June 1813 https://founders.archives.gov/documents/Jefferson/03-06-02-0188; though most publications of the letter since the 1830s usually provide a date of 18 June 1813, the actual manuscript seems to distinctly read "June 19 '13" http://memory.loc.gov/master/mss/mtj/mtj1/046/0800/0894.jpg; a portion of this statement is sometimes paraphrased: "Every citizen should be a soldier."
1810s
Context: !-- Dear Sir,—Your favors of the 7th and 16th are received, and --> I now return you the memoir … I am much gratified by its communication, because, as the plan appeared in the newspapers soon after the new Secretary of War came into office, we had given him the credit of it. Every line of it is replete with wisdom; and we might lament that our tardy enlistments prevented its execution, were we not to reflect that these proceeded from the happiness of our people at home. It is more a subject of joy that we have so few of the desperate characters which compose modern regular armies. But it proves more forcibly the necessity of obliging every citizen to be a soldier; this was the case with the Greeks and Romans, and must be that of every free State. Where there is no oppression there will be no pauper hirelings. We must train and classify the whole of our male citizens, and make military instruction a regular part of collegiate education. We can never be safe till this is done.

The Notebooks of Leonardo da Vinci (1883), XXI Letters. Personal Records. Dated Notes.