The Perpetual Union is a feature of the Articles of Confederation and Perpetual Union, which established the United States of America as a national entity. Under modern American constitutional law, this concept means that U.S. states are not permitted to overthrow the U.S. Constitution and withdraw from the Union.
The Union is much older than the Constitution. It was formed, in fact, by the Articles of Association in 1774. It was matured and continued by the Declaration of Independence in 1776. It was further matured, and the faith of all the then thirteen States expressly plighted and engaged that it should be perpetual, by the Articles of Confederation in 1778. And finally, in 1787, one of the declared objects for ordaining and establishing the Constitution was to form a more perfect Union. 
A significant step was taken on June 12, 1776, when the Second Continental Congress approved the drafting of the Articles of Confederation, following a similar approval to draft the Declaration of Independence on June 11. The purpose of the former document was not only to define the relationship among the new states but also to stipulate the permanent nature of the new union. Accordingly, Article XIII states that the Union "shall be perpetual". While the process to ratify the Articles began in 1777, the Union only became a legal entity in 1781 when all states had ratified the agreement. The Second Continental Congress approved the Articles for ratification by the sovereign States on November 15, 1777, which occurred during the period from July 1778 to March 1781.
The 13th ratification by Maryland was delayed for several years due to conflict of interest with some other states, including the western land claims of Virginia. After Virginia passed a law on January 2, 1781 relinquishing the claims, the path forward was cleared. On February 2, 1781, the Maryland state legislature in Annapolis passed the Act to ratify and on March 1, 1781 the Maryland delegates to the Second Continental Congress in Philadelphia formally signed the agreement. Maryland's final ratification of the Articles of Confederation and perpetual Union established the requisite unanimous consent for the legal creation of the United States of America.
The concept of a perpetual union appeared earlier in European political thought. In 1532, Francis I of France signed the Treaty of Perpetual Union (fr. Traité d'Union Perpétuelle), which pledged the freedom and privileges of the Duchy of Brittany within the Kingdom of France.  In 1713, Charles de Saint-Pierre presented a plan "A project for settling an everlasting peace in Europe," where in it is stated in Article 1:
There shall be from this day following a Society, a permanent and perpetual Union, between the Sovereigns subscribed." 
By itself the word perpetual appears much earlier in the history of political thought. In January 44 B.C., Denarii coins were struck with the image of Julius Caesar and the Latin inscription "Caesar Dic(tator in) Perpetuo". 
From the start the Union has carried with it importance in the national affairs. There was a sense of urgency in completing the legal Union during the American Revolutionary War. Maryland's ratification act stated, "[I]t hath been said that the common enemy is encouraged by this State not acceding to the Confederation, to hope that the union of the sister states may be dissolved"  The nature of the Union was hotly debated during a period lasting from the 1830s through the American Civil War. During the American Civil War, the U.S. was called "the Union".
When the United States Constitution replaced the Articles, nothing in it expressly stated that the Union is perpetual. Even after the Civil War, which had been fought by the U.S. to prevent the southern slave states from overthrowing the Constitution and leaving the Union, some still questioned whether any such inviolability survived after the U.S. Constitution replaced the Articles. This uncertainty also stems from the fact that the Constitution, technically an amendment of the perpetual Articles, was not ratified unanimously before going into effect, as required by the Articles (two states, North Carolina and Rhode Island, had not ratified and remained outside the Union when George Washington was sworn in as the first U.S. President). The United States Supreme Court ruled on the issue in the 1869 Texas v. White case.  In that case, the court ruled that the drafters intended the perpetuity of the Union to survive:
By [the Articles of Confederation], the Union was solemnly declared to "be perpetual." And when these Articles were found to be inadequate to the exigencies of the country, the Constitution was ordained "to form a more perfect Union." It is difficult to convey the idea of indissoluble unity more clearly than by these words. What can be indissoluble if a perpetual Union, made more perfect, is not?— U.S. Supreme Court, Texas v. White (1869). 
During the ratification of the Constitution, ratifications by New York, Virginia and Rhode Island included language that reserved the right of those states to exit the U.S. federal system if they felt "harmed" by the arrangement. In Virginia's ratification the reservation is stated thus; "  the People of Virginia declare and make known that the powers granted under the Constitution being derived from the People of the United States may be resumed by them whensoever the same shall be perverted to their injury or oppression ..."
My opinion is that a reservation of a right to withdraw ... is a conditional ratification ... Compacts must be reciprocal ... The Constitution requires an adoption in toto, and for ever. It has been so adopted by the other States.
Hamilton and John Jay agreed with Madison's view, reserving "a right to withdraw [was] inconsistent with the Constitution, and was no ratification."  The New York convention ultimately ratified the Constitution without including the "right to withdraw" language proposed by the anti-federalists. 
In his first inaugural address, George Washington referred to an "indissoluble union", and in his farewell address to the country, telling Americans that they should maintain "the security of their union and the advancement of their happiness."  In his farewell address, Washington stated that the union of states was "your union and brotherly affection may be perpetual", and in urging Americans to maintain it, stated that "you should properly estimate the immense value of your national Union to your collective and individual happiness."  Patrick Henry, shortly before his death, urged Americans not to "split into factions which must destroy that union upon which our existence hangs." 
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