"We, the People of the State of Texas, by Delegates in Convention assembled, do declare and ordain, that the Ordinance adopted by our Convention of Delegates, on the Fourth day of July, A.D. 1845, and afterwards ratified by us, under which the Republic of Texas was admitted into Union with other States and became a party to the compact styled "The Constitution of the United States of America" be, and is hereby repealed and annulled; That all the powers, which by said compact were delegated by Texas to the Federal Government, are revoked and resumed; That Texas is of right absolved from all restraints and obligations incurred by said compact, and is a separate Sovereign State, and that her citizens and people are absolved from all allegiance to the United States, or the Government thereof."
The exact same way they entered the union and it was the same for all the others as well!
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, anything in the constitution or laws of any state to the contrary notwithstanding. - The Supremacy Clause
The Constitution was the restatement of the Articles of Confederation. The Union was/is perpetual and indissoluble.
If Texans thought they could enter the Union at will and leave it in the same fashion, they were wrong.
If they think they can still do it today, they are still wrong.
U.S. Constitution, Article III, Section 2:
"The judicial power shall extend to all cases, in law and equity, arising under this Constitution, the laws of the United States, and treaties made, or which shall be made, under their authority;--to all cases affecting ambassadors, other public ministers and consuls;--to all cases of admiralty and maritime jurisdiction;--to controversies to which the United States shall be a party."The United States was a party in Texas v. White.
"The Union of the States never was a purely artificial and arbitrary relation. It began among the Colonies, and grew out of common origin, mutual sympathies, kindred principles, similar interests, and geographical relations. It was confirmed and strengthened by the necessities of war, and received definite form and character and sanction from the Articles of Confederation. By these, 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?" - Texas v. White, April 12, 1869 Chief Justice Salmon Chase writing for the majority
Secession, whatever anyone else believed then or believes today to the contrary, was/is unconstitutional.
The Union can be only dissolved by the will of
all its members, but no member shall establish an independent nation within the boundaries of the United States.
Period.