Federalist No. 9 can be almost completely reduced to this extended quote by Montesquieu:
a CONFEDERATE REPUBLIC. . . is a convention by which several smaller STATES agree to become members of a larger ONE, which they intend to form. It is a kind of assemblage of societies that constitute a new one, capable of increasing, by means of new associations, till they arrive to such a degree of power as to be able to provide for the security of the united body. . .
The form of this society prevents all manner of inconveniences.
If a single member should attempt to usurp the supreme authority, he could not be supposed to have an equal authority and credit in all the confederate states. Were he to have too great influence over one, this would alarm the rest. Were he to subdue a part, that which would still remain free might oppose him with forces independent of those which he had usurped and overpower him before he could be settled in his usurpation.
Should a popular insurrection happen in one of the confederate states the others are able to quell it. Should abuses creep into one part, they are reformed by those that remain sound. The state may be destroyed on one side, and not on the other; the confederacy may be dissolved, and the confederates preserve their sovereignty. (“Spirit of Lawa,” vol. i., book ix., chap. i.)
Ever since the United States made the transition from being a plural noun (ex. “the United States are . . .”) to being a singular noun (ex. “the United States is . . .”) it seems that the individual sovereignty and identity of the states is becoming more of a formality than a reality. Contrary to the argument made in Federalist 9, I am not convinced that “So long as the separate organization of the members be not abolished; so long as it exists, by a constitutional necessity, for local purposes; though it should be in perfect subordination to the general authority of the union, it would still be, in fact and in theory, an association of states, or a confederacy.”
It seems that the last half century or more have shown that when the states become fully subordinate to the federal government the balancing of interests that are present in a confederacy are minimized and the Federal authority begins to act more like a monarchy with an ever-changing head.