Blackstone and Madison on League vs. Union

William Blackstone, Commentaries on the Laws of England (1765-69), discussing the Act of Union of 1707, by which England and Scotland united to form Great Britain

UPON these articles, and act of union, it is to be observed, 1. That the two kingdoms are now so inseparably united, that nothing can ever disunite them again, but an infringment of those points which, when they were separate and independent nations, it was mutually stipulated should be “fundamental and “essential conditions of the union.”

It may justly be doubted, whether even such an infringement (though a manifest breach of good faith, unless done upon the most pressing necessity) would consequentially dissolve the union: for the bare idea of a state, without a power somewhere vested to alter every part of its laws, is the height of political absurdity. The truth seems to be, that in such an incorporate union (which is well distinguished by a very learned prelate from a foederate alliance, where such an infringement would certainly rescind the compact) the two contracting states are totally annihilated, without any power of revival; and a third arises from their conjunction, in which all the rights of sovereignty, and particularly that of legislation, must of necessity reside. (See Warburton's alliance. 195.) But the imprudent exertion of this right would probably raise a very alarming ferment in the minds of individuals; and therefore it is hinted above that such an attempt might endanger (though not certainly destroy) the union.

Max Farrand, Records of the Federal Convention of 1787, at 2:93 (July 23, 1787):

[James Madison] considered the difference between the system founded on the Legislatures only, and one founded on the people to be the true difference between a league or treaty, and a Constitution. The former in point of moral obligation might be as inviolable as the latter. In point of political operation, there were two important distinctions in favor of the latter. 1 . . .. 2. The doctrine laid down by the law of Nations in the case of treaties is that a breach of any one article by any of the parties, frees the other parties from their engagements. In the case of a union of people under one Constitution, the nature of the pact has always been understood to exclude such an interpretation.