"The unity of government which constitutes you one people is also now dear to you. It is justly so, for it is a main pillar in the edifice of your real independence, the support of your tranquility at home, your peace abroad; of your safety; of your prosperity; of that very liberty which you so highly prize. But as it is easy to foresee that, from different causes and from different quarters, much pains will be taken, many artifices employed to weaken in your minds the conviction of this truth; as this is the point in your political fortress against which the batteries of internal and external enemies will be most constantly and actively (though often covertly and insidiously) directed, it is of infinite moment that you should properly estimate the immense value of your national union to your collective and individual happiness; that you should cherish a cordial, habitual, and immovable attachment to it; accustoming yourselves to think and speak of it as of the palladium of your political safety and prosperity; watching for its preservation with jealous anxiety; discountenancing whatever may suggest even a suspicion that it can in any event be abandoned; and indignantly frowning upon the first dawning of every attempt to alienate any portion of our country from the rest, or to enfeeble the sacred ties which now link together the various parts.For this you have every inducement of sympathy and interest. Citizens, by birth or choice, of a common country, that country has a right to concentrate your affections. The name of American, which belongs to you in your national capacity, must always exalt the just pride of patriotism more than any appellation derived from local discriminations. With slight shades of difference, you have the same religion, manners, habits, and political principles. You have in a common cause fought and triumphed together; the independence and liberty you possess are the work of joint counsels, and joint efforts of common dangers, sufferings, and successes.
But these considerations, however powerfully they address themselves to your sensibility, are greatly outweighed by those which apply more immediately to your interest. Here every portion of our country finds the most commanding motives for carefully guarding and preserving the union of the whole."
-- George Washington, Farewell Address
“The original states were created at the same moment as the nation, at the signing of the Declaration of Independence, an act that was carried out, and later enforced, by the representatives of the whole people, of all the former British colonies.”
The Declaration of Independence didn’t form any nation. It simply declared the independence of the colonies from Britain. It was not until the Articles of Confederation that a nation was formed.
The states existed as soon as they declared Independence, since they were no longer colonies. They could not have formed a nation amongst themselves subsequently if they were not, at that point, sovereign states, because they would have no power to do such a thing.
Now, you can argue that states which came along after the original 13 were formed under the authority of the United States government, but they were still formed as states, co-equal with the original 13 states, and therefore possessing all the same rights as those 13. Since the 10th amendment guarantees states all rights not specifically assigned to the federal government, and the federal government was never assigned the rights of self-governance or self-determination, then those rights belong to the newer states, under the 10th amendment, just as they still belong to the original 13.
I would maintain that the cited Farewell Address was a statement of the theory. The Civil War was putting that theory to practice.