Federalist No. 25

In Federalist No. 25, Hamilton continues his consideration of the powers necessary to the common defense, which he began in the last instalment where he determined that restraints upon the discretion of the legislature in respect to military establishments in time of peace, would be improper to be imposed as here has been a constant necessity for keeping small garrisons on our Western frontier and that alone justifies the need for standing armies, even in peace-time.

In this piece he noted that the territories of Britain, Spain, and of the Indian nations … encircle the Union from Maine to Georgia and that the danger, though in different degrees, is therefore common. And the means of guarding
against it ought, in like manner, to be the objects of common councils and of a common treasury

If power was instead to be imbued in State governments, rivalships could form, due to the inherent love of power possessed by any assembly, and in any contest between the federal head and one of its members, the people will be most apt to unite with their local government. Plus, if the ambition of the members should be stimulated by the separate and independent possession of military forces,
it would afford too strong a temptation and too great a facility to them to make enterprises upon, and finally to subvert, the
constitutional authority of the Union
. As a result, the liberty of the people would be less safe in this state of things
than in that which left the national forces in the hands of the national government

And if the need for a national army under the control of the federal government is not yet clear, we can obviate this consequence by noting that the United States would then exhibit the most extraordinary spectacle which the world has yet seen, that of a nation
incapacitated by its Constitution to prepare for defense, before it was actually invaded