"Now, for what you have all been waiting for, the legal analysis: At the culmination of the Civil War, the United States Supreme Court decided Texas v. White, which started as a dispute over bonds issued in Texas during the war. Texas attempted to leave the United States when it supported the Confederacy, where it supplied troops to fight with the rebel forces. The United States Supreme Court established a new constitutional principle in Texas v. White, holding that states cannot unilaterally secede.

Chief Justice Salmon P. Chase stated: “When Texas became one of the United States, she entered into an indissoluble relation. All the obligations of perpetual union, and all the guaranties of republican government in the Union, attached at once to the state. The Act, which consummated her admission into the Union, was something more than a compact; it was the incorporation of a new member into the political body. And it was final.”

If it doesn’t seem hard enough to secede, some constitutional scholars say that secession could be considered treason under the Articles of the Constitution. In an article for the Kansas City Star, Akhil Reed Amar, Sterling Professor of Law and Political Science at Yale University, discussed the Supremacy Clause in Article 6 of the United States Constitution.

“What the Constitution says repeatedly is once you’re in (as a state), you’re in. If people want to secede, they are allowed to leave; they just can’t take the land and the water with them. There is a lawful way to secede – it’s called emigration. They can move to Canada,” Amar wrote."

https://sites.law.duq.edu/juris/2012/12/11/can-a-state-legally-secede-from-the-united-states/