It sounds quaint, but a union volunterily entered, can and should be able to be volunterily left, short of murdering your spouse or a civil war IMO.
That's up to the members of the Union to collectively decide. The states fought for and won their sovereignty, they could do what they wanted with it. They could establish a Union which which they could voluntarily enter and, via the retention of full sovereignty, they could voluntarily leave. And if that Union failed for the same reason for which they could voluntarily leave, the retention of full sovereignty, they could have decided to form and voluntarily enter a union from which individual states could NOT legally withdraw unilaterally, via the surrender of a portion of their sovereignty. They could have felt that the Union was so important to their individual survival that receiving that level of commitment and obligation FROM the other states was worth making that level of commitment and obligation TO the other states?
The Constitution is a fundamental law enacted directly by the people of the United States, and therefore represents a social compact as per Locke, not a compact among sovereign states that remain fully sovereign (a treaty, as per Vattel). When any group of people execute a social compact, they are performing a sovereign act, and become one sovereign people.
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'We the people of the United States, do ordain and establish this Constitution.' Here we see the people acting as sovereigns of the whole country; and in the language of sovereignty, establishing a Constitution...” – SCOTUS, Chisholm v. Georgia, 1793
In a political society established by social compact, only the sovereign people have an inherent sovereign right/power to alter or abolish the government and society they created. And when more than one person is the sovereign, they must of course act by majority rule. No minority segment of the sovereign people has any right to alter or abolish the fundamental law OR the political state it in turn establishes, unless and except as specifically granted by the sovereign people via their fundamental law. All minority segments of such a society are legally bound to the fundamental law and the will of a majority of the sovereign people. Placing the Union on its own foundation of sovereignty (that of the people of the US) legally binds the states to the Union.
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by which it was their will, that the State Governments should be bound, and to which the State Constitutions should be made to conform.” – Ibid
Sovereignty constitutes supreme control of a political state.
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Sovereignty: the supreme, absolute, and uncontrollable power by which an independent state is governed and from which all specific political powers are derived;...” – Legal Dictionary at thefreedictionary,com
A social compact, fundamental law, and sovereignty at the Union level work the same way for the Union as they do for the states.
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Every State Constitution is a compact made by and between the citizens of a State to govern themselves in a certain manner; and the Constitution of the United States is likewise a compact made by the people of the United States to govern themselves as to general objects, in a certain manner.” – Chisholm v. Georgia
However, for those who may not understand sovereignty, the Constitution makes it crystal clear:
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This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any thing in the Constitution or Laws of any State to the Contrary notwithstanding.”
The unique thing about the Constitution was that the people performing this sovereign act were previously arranged in 13 separately sovereign states (nations), and the national fundamental law represented limited sovereignty as per the principle of limited, enumerated powers. The former means the states were surrendering sovereignty to the people of the US. The latter means they were only surrendering a portion of their previously full sovereignty (the portion associated with the Constitution). They did this because the Confederation, a union in which each state retains full sovereignty (which secures all rights of independent sovereignty to each state), had proven that such was untenable. The state legislatures blew it; they failed to voluntarily comply with the rules when the rules required only voluntary compliance. As a social compact among the people of the states, the creation of the Constittuion works much like the classic, theoretical social compact, in which individuals in a state of nature, where each individual has the right/liberty to apply natural law, gives up that liberty in order to preserve their natural, inalienable rights (life, liberty, and the pursuit of happiness) by forming and entering a political society.
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It is obviously impracticable in the federal government of these states, to secure all rights of independent sovereignty to each, and yet provide for the interest and safety of all: Individuals entering into society, must give up a share of liberty to preserve the rest.” – Official Letter of the Federal Convention of 1787, September 17, 1787
The Constitution therefore created not just another union, but a union that consists of a sovereign nation in which the sovereignty that makes a political state fully independent is divided between the Union itself and the individual states of the Union.
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It has hitherto been understood, that the supreme power, that is, the sovereignty of the people of the States, was in its nature divisible; and was in fact divided, according to the Constitution of the U. States, between the States in their United, and the States in their individual capacities.” – Madison, Essay on Sovereignty, December 1835
Being legally supreme, absolute, and uncontrollable, the only legal transfer of sovereignty is a consensual one. This is the reason why the decision to enact and adopt the Constitution had to be made 1) by the sovereign people of the states and not their governments, and 2) by each formerly fully sovereign state separately. And any return of that sovereignty by the people of the United States to the people of one or a number of individual states must similarly be consensual. Therefore, legally speaking, a sovereign state is legally indissoluble (except by the sovereign). The AoCs established a Union deemed “perpetual” by treaty, but the Constitution established a Union that was legally indissoluble, just like the sovereign states themselves had been. The Union became perpetual by fundamental law. In 1783, while leaving office as the general of the Continental Army, Washington said:
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There are four things, which I humbly conceive are essential to the well being, I may even venture to say to the existence, of the United States as an independent Power. 1st An indissoluble Union of the States under one federal Head.” – George Washington, Circular Letter to the States, June 8, 1783
And in 1796, in his Farewell Address, he indicated that the Union under the Constitution had in fact become indissoluble.
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The West derives from the East supplies requisite to its growth and comfort, and, what is perhaps of still greater consequence, it must of necessity owe the secure enjoyment of indispensable outlets for its own productions to the weight, influence, and the future maritime strength of the Atlantic side of the Union, directed by an indissoluble community of interest as one nation.” – George Washington, Farewell Address, September 19, 1796
In the Federalist No. 5, John Jay identified the indissoluble union between England and Scotland as an example of what the Americans should create via the Constitution:
“QUEEN ANNE, in her letter of the 1st July, 1706, to the Scotch Parliament, makes some observations on the importance of the UNION then forming between England and Scotland, which merit our attention. I shall present the public with one or two extracts from it: ‘
An entire and perfect union will be the solid foundation of lasting peace: It will secure your religion, liberty, and property; remove the animosities amongst yourselves, and the jealousies and differences betwixt our two kingdoms. It must increase your strength, riches, and trade; and by this union the whole island, being joined in affection and free from all apprehensions of different interest, will be ENABLED TO RESIST ALL ITS ENEMIES. We most earnestly recommend to you calmness and unanimity in this great and weighty affair, that the union may be brought to a happy conclusion, being the only EFFECTUAL way to secure our present and future happiness, and disappoint the designs of our and your enemies, who will doubtless, on this occasion, USE THEIR UTMOST ENDEAVORS TO PREVENT OR DELAY THIS UNION.’ ” (all-caps emphasis either hers or his)
And in the Federalist No. 11, Hamilton said:
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Let the thirteen States, bound together in a strict and indissoluble Union, concur in erecting one great American system, superior to the control of all transatlantic force or influence, and able to dictate the terms of the connection between the old and the new world!”
Placing the Union on its own foundation of sovereignty therefore consolidated the Union by making it perpetual AND indissoluble.
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In all our deliberations on this subject we kept steadily in our view, that which appears to us the greatest interest of every true American, the consolidation of our Union, in which is involved our prosperity, felicity, safety, perhaps our national existence.” – Official Letter of the Federal Convention of 1787, September 17, 1787
Put this all together, and what do you have:
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Under the Articles of Confederation each State retained its sovereignty, freedom, and independence, and every power, jurisdiction, and right not expressly delegated to the United States. Under the Constitution, though the powers of the States were much restricted, still, all powers not delegated to the United States, nor prohibited to the States, are reserved to the States respectively, or to the people. And we have already had occasion to remark at this term, that the people of each State compose a State, having its own government, and endowed with all the functions essential to separate and independent existence,' and that 'without the States in union, there could be no such political body as the United States.' Not only, therefore, can there be no loss of separate and independent autonomy to the States, through their union under the Constitution, but it may be not unreasonably said that the preservation of the States, and the maintenance of their governments, are as much within the design and care of the Constitution as the preservation of the Union and the maintenance of the National government. The Constitution, in all its provisions, looks to an indestructible Union, composed of indestructible States.” – SCOTUS, Texas v. White, 1869