Article 2, Section 2, Clause 1

Document 5

Alexander Hamilton, Federalist, no. 74, 500

The President of the United States is to be "Commander in Chief of the army and navy of the United States, and of the militia of the several States when called into the actual service of the United States." The propriety of this provision is so evident in itself; and it is at the same time so consonant to the precedents of the State constitutions in general, that little need be said to explain or enforce it. Even those of them, which have in other respects coupled the Chief Magistrate with a Council, have for the most part concentred the military authority in him alone. Of all the cares or concerns of government, the direction of war most peculiarly demands those qualities which distinguish the exercise of power by a single hand. The direction of war implies the direction of the common strength; and the power of directing and employing the common strength, forms an usual and essential part in the definition of the executive authority.

The Founders' Constitution
Volume 4, Article 2, Section 2, Clause 1, Document 5
http://press-pubs.uchicago.edu/founders/documents/a2_2_1s5.html
The University of Chicago Press

Hamilton, Alexander; Madison, James; and Jay, John. The Federalist. Edited by Jacob E. Cooke. Middletown, Conn.: Wesleyan University Press, 1961.