File this under the “Founding Fathers hit the nail on the head” category. From Federalist No. 23 by Alexander Hamilton (he who graces the $10 bill).
The principal purposes to be answered by union are these– the common defense of the members; the preservation of the public peace as well against internal convulsions as external attacks; the regulation of commerce with other nations and between the States; the superintendence of our intercourse, political and commercial, with foreign countries.
The authorities essential to the common defense are these: to raise armies; to build and equip fleets; to prescribe rules for the government of both; to direct their operations; to provide for their support. These powers ought to exist without limitation, because it is impossible to foresee or define the extent and variety of national exigencies, or the correspondent extent and variety of the means which may be necessary to satisfy them. The circumstances that endanger the safety of nations are infinite, and for this reason no constitutional shackles can wisely be imposed on the power to which the care of it is committed. This power ought to be coextensive with all the possible combinations of such circumstances; and ought to be under the direction of the same councils which are appointed to preside over the common defense.
Emphasis Original.