Article 2, Section 2, Clause 1
William Blackstone, Commentaries 1:2541765
II. The king is considered, in the next place, as the generalissimo, or the first in military command, within the kingdom. The great end of society is to protect the weakness of individuals by the united strength of the community: and the principle use of government is to direct that united strength in the best and most effectual manner, to answer the end proposed. Monarchical government is allowed to be the fittest of any for this purpose: it follows therefore, from the very end of it's institution, that in a monarchy the military power must be trusted in the hands of the prince.
The Founders' Constitution
Volume 4, Article 2, Section 2, Clause 1, Document 2
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.
Easy to print version.