What does the word "union" mean to you? Not like the ex Soviet Union where Russia simply told a bunch of other countries they were in. When we formed the United States of America, the states voluntarily gave up their right as sovereign entities....which they were for only a very brief period of time after the war.
Now you can find all sort of phrases or quote words from different people as to what the Founders "really meant," but the fact is if they wanted the states to be able to leave at will they would have very clearly put it in writing...in the constitution. They didn't. Because it would have been insane and the end of the union.
The Constitution was the vehicle by which those sovereign states granted power to a federal entity for the purposes I have outlined (which the founders specifically outlined, too). The purpose wasn't to cede all power to a National Government and leave the scraps for the States but to give narrowly defined roles to the Federal Government and Retain the vast bulk of power to the States and the People. That is stated repeatedly in the document and the Bill of Rights. It requires no more interpretation than a fundamental understanding of the English language.
Why would a Governor, a Legislature, the entire court system, the landholders, the People of a sovereign State throw all their self-determination, power and Liberty to a small group of people with conflicting interests one, maybe two weeks by land or a long boat ride away when they had just fought to eliminate that entire situation of being governed from afar?
The Constitution would never have been ratified had that been the case. The harping about
"The Union" . is a political artifact of the War of Northern Aggression, where the "Union" was used along with Abolitionist propaganda to justify forcing at gunpoint the return of the Southern agricultural producing areas northern textile mills heavily exploited to the purview of the heavily Northern dominated Congress and the cronies thereof. You won't see such ballyhoo about "Union" in the Federalist papers, and it is only used in the preamble in the sense of forming a Federation, not a National Government.
Otherwise, of what use would have been the Secretary of State, Attorney General, Supreme Courts, and State Legislatures (to mention a few) and other associated offices which perform the functions of those of a nation, but are found in each State? Why did the States have their several Constitutions, when one would suffice for a Nation united: a Union--in a time when many well established Nations did not have a Constitution?
Those offices and Constitutions are suitable for nations, not mere fiefdoms to rubber stamp the edicts from afar. The concentration of power we see today in DC was NOT the intent of the Founders, the original power resided with The People, who granted powers to their States, who grudgingly ceded only such power as was necessary to keep a Federation viable, for the benefit of mutual defense, coining money, negotiating with foreign powers, and to settle squabbles among the several States.