Article 1, Section 8, Clause 15

Document 1

William Blackstone, Commentaries 1:395

In a land of liberty it is extremely dangerous to make a distinct order of the profession of arms. In absolute monarchies this is necessary for the safety of the prince, and arises from the main principle of their constitution, which is that of governing by fear: but in free states the profession of a soldier, taken singly and merely as a profession, is justly an object of jealousy. In these no man should take up arms, but with a view to defend his country and it's laws: he puts not off the citizen when he enters the camp; but it is because he is a citizen, and would wish to continue so, that he makes himself for a while a soldier. The laws therefore and constitution of these kingdoms know no such state as that of a perpetual standing soldier, bred up to no other profession than that of war: and it was not till the reign of Henry VII, that the kings of England had so much as a guard about their persons.

The Founders' Constitution
Volume 3, Article 1, Section 8, Clause 15, Document 1
http://press-pubs.uchicago.edu/founders/documents/a1_8_15s1.html
The University of Chicago Press

Blackstone, William. Commentaries on the Laws of England: A Facsimile of the First Edition of 1765--1769. Chicago: University of Chicago Press, 1979.

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