No it wasn’t legal, it was treason. There was precedent for this from a previous President.
The States severally have not retained their entire sovereignty. It has been shown that in becoming parts of a nation, not members of a league, they surrendered many of their essential parts of sovereignty. The right to make treaties, declare war, levy taxes, exercise exclusive judicial and legislative powers, were all functions of sovereign power. The States, then, for all these important purposes, were no longer sovereign. The allegiance of their citizens was transferred in the first instance to the government of the United States; they became American citizens, and owed obedience to the Constitution of the United States, and to laws made in conformity with the powers vested in Congress. This last position has not been, and cannot be, denied… it has been shown that in this sense the States are not sovereign, and that even if they were, and the national Constitution had been formed by compact, there would be no right in any one State to exonerate itself from the obligation.
So obvious are the reasons which forbid this secession, that it is necessary only to allude to them. The Union was formed for the benefit of all. It was produced by mutual sacrifice of interest and opinions. Can those sacrifices be recalled? Can the States, who magnanimously surrendered their title to the territories of the West, recall the grant? Will the inhabitants of the inland States agree to pay the duties that may be imposed without their assent by those on the Atlantic or the Gulf, for their own benefit? Shall there be a free port in one State, and enormous duties in another? No one believes that any right exists in a single State to involve all the others in these and countless other evils, contrary to engagements solemnly made. Everyone must see that the other States, in self-defense, must oppose it at all hazards.
Your pride was aroused by the assertions that a submission to these laws was a state of vassalage, and that resistance to them was equal, in patriotic merit, to the opposition our fathers offered to the oppressive laws of Great Britain. You were told that this opposition might be peaceably-might be constitutionally made-that you might enjoy all the advantages of the Union and bear none of its burdens. Eloquent appeals to your passions, to your State pride, to your native courage, to your sense of real injury, were used to prepare you for the period when the mask which concealed the hideous features of DISUNION should be taken off.
But the dictates of a high duty oblige me solemnly to announce that you cannot succeed. The laws of the United States must be executed. I have no discretionary power on the subject-my duty is emphatically pronounced in the Constitution. Those who told you that you might peaceably prevent their execution, deceived you-they could not have been deceived themselves. They know that a forcible opposition could alone prevent the execution of the laws, and they know that such opposition must be repelled. Their object is disunion, but be not deceived by names; disunion, by armed force, is TREASON. Are you really ready to incur its guilt?
President Jackson’s Proclamation Regarding Nullification, December 10, 1832
“Americans celebrate Abraham Lincoln’s Gettysburg Address, but H.L. Mencken correctly evaluated the speech, “It is poetry not logic; beauty, not sense.” Lincoln said that the soldiers sacrificed their lives “to the cause of self-determination — government of the people, by the people, for the people should not perish from the earth.” Mencken says: “It is difficult to imagine anything more untrue. The Union soldiers in the battle actually fought against self-determination; it was the Confederates who fought for the right of people to govern themselves.”
https://www.capitalismmagazine.com/2002/04/do-states-have-a-right-of-secession/