I don't see how it could possibly be considered "legal" to rip a chunk out of a country. Expecting them to be cool with that is to ignore all of human history.
https://en.wikipedia.org/wiki/Secession ... _secession
Dubious legality of unilateral secession
The Constitution does not directly mention secession.[53] The legality of secession was hotly debated in the 19th century, with Southerners often claiming and Northerners generally denying that states have a legal right to unilaterally secede.[54] The Supreme Court has consistently interpreted the Constitution to be an "indestructible" union.[53] There is no legal basis a state can point to for unilaterally seceding.[55] Many scholars hold that the Confederate secession was blatantly illegal. The Articles of Confederation explicitly state the Union is "perpetual"; the U.S. Constitution declares itself an even "more perfect union" than the Articles of Confederation.[56] Other scholars, while not necessarily disagreeing that the secession was illegal, point out that sovereignty is often de facto an "extralegal" question. Had the Confederacy won, any illegality of its actions under U.S. law would have been rendered irrelevant, just as the undisputed illegality of American rebellion under the British law of 1775 was rendered irrelevant. Thus, these scholars argue, the illegality of unilateral secession was not firmly de facto established until the Union won the Civil War; in this view, the legal question was resolved at Appomattox.[54][57]
Supreme Court rulings
Texas v. White[56] was argued before the United States Supreme Court during the December 1868 term. Chief Justice Salmon P. Chase read the Court's decision, on April 15, 1869.[58] Australian Professors Peter Radan and Aleksandar Pavkovic write:
Chase, [Chief Justice], ruled in favor of Texas on the ground that the Confederate state government in Texas had no legal existence on the basis that the secession of Texas from the United States was illegal. The critical finding underpinning the ruling that Texas could not secede from the United States was that, following its admission to the United States in 1845, Texas had become part of "an indestructible Union, composed of indestructible states". In practical terms, this meant that Texas has never seceded from the United States.[59]
However, the Court's decision recognized some possibility of the divisibility "through revolution, or through consent of the States".[59][60]
In 1877, the Williams v. Bruffy[61] decision was rendered, pertaining to civil war debts. The Court wrote regarding acts establishing an independent government that "The validity of its acts, both against the parent state and the citizens or subjects thereof, depends entirely upon its ultimate success; if it fail to establish itself permanently, all such acts perish with it; if it succeed and become recognized, its acts from the commencement of its existence are upheld as those of an independent nation."[59][62]
Basically, "if you win, you can secede".
The Union as a sovereign state
Historian Kenneth Stampp notes that a historical case against secession had been made that argued that "the Union is older than the states" and that "the provision for a perpetual Union in the Articles of Confederation" was carried over into the Constitution by the "reminder that the preamble to the new Constitution gives us one of its purposes the formation of 'a more perfect Union'".[20] Concerning the White decision Stampp wrote:
In 1869, when the Supreme Court, in Texas v. White, finally rejected as untenable the case for a constitutional right of secession, it stressed this historical argument. The Union, the Court said, "never was a purely artificial and arbitrary relation". Rather, "It began among the Colonies. ...It was confirmed and strengthened by the necessities of war, and received definite form, and character, and sanction from the Articles of Confederation."[20]