Article 4, Section 4

Document 8

James Madison, Virginia Ratifying Convention

But the honorable member sees great danger in the provision concerning the militia. This I conceive to be an additional security to our liberty, without diminishing the power of the states in any considerable degree. It appears to me so highly expedient that I should imagine it would have found advocates even in the warmest friends of the present system. The authority of training the militia, and appointing the officers, is reserved to the states. Congress ought to have the power to establish a uniform discipline throughout the states, and to provide for the execution of the laws, suppress insurrections, and repel invasions: these are the only cases wherein they can interfere with the militia; and the obvious necessity of their having power over them in these cases must convince any reflecting mind. Without uniformity of discipline, military bodies would be incapable of action: without a general controlling power to call forth the strength of the Union to repel invasions, the country might be overrun and conquered by foreign enemies: without such a power to suppress insurrections, our liberties might be destroyed by domestic faction, and domestic tyranny be established.

The Founders' Constitution
Volume 4, Article 4, Section 4, Document 8
http://press-pubs.uchicago.edu/founders/documents/a4_4s8.html
The University of Chicago Press

Elliot, Jonathan, ed. The Debates in the Several State Conventions on the Adoption of the Federal Constitution as Recommended by the General Convention at Philadelphia in 1787. . . . 5 vols. 2d ed. 1888. Reprint. New York: Burt Franklin, n.d.