Article 4, Section 4
St. George Tucker, Blackstone's Commentaries 1:App. 366--671803
It is an observation of the enlightened Montesquieu, that mankind would have been at length obliged to submit to the government of a single person, if they had not contrived a kind of constitution, by which the internal advantages of a republic might be united with the external force of a monarchy; and this constitution is that of a confederacy of smaller states, to form one large one for their common defence. But these associations ought only to be formed, he tells us, between states whose form of government is not only similar, but also republican. The spirit of monarchy is war, and the enlargement of dominion; peace and moderation is the spirit of a republic. These two kinds of government cannot naturally subsist together in a confederate republic. Greece, he adds, was undone, as soon as the kings of Macedon obtained a seat among the Amphictions. If the United States wish to preserve themselves from a similar fate, they will consider the guarantee contained in this clause as a corner stone of their liberties.
The possibility of an undue partiality in the federal government in affording it's protection to one part of the union in preference to another, which may be invaded at the same time, seems to be provided against, by that part of this clause which guarantees such protection to each of them. So that every state which may be invaded must be protected by the united force of the confederacy. It may not be amiss further to observe, that every pretext for intermeddling with the domestic concerns of any state, under colour of protecting it against domestic violence is taken away, by that part of the provision which renders an application from the legislative, or executive authority of the state endangered, necessary to be made to the federal government, before it's interference can be at all proper. On the other hand, this article secures an immense acquisition of strength; and additional force to the aid of any of the state governments, in case of an internal rebellion or insurrection against it's authority. . . . The southern states being more peculiarly open to danger from this quarter, ought to be particularly tenacious of a constitution from which they may derive such assistance in the most critical periods.
The Founders' Constitution
Volume 4, Article 4, Section 4, Document 11
The University of Chicago Press
Tucker, St. George. Blackstone's Commentaries: With Notes of Reference to the Constitution and Laws of the Federal Government of the United States and of the Commonwealth of Virginia. 5 vols. Philadelphia, 1803. Reprint. South Hackensack, N.J.: Rothman Reprints, 1969.