People were free to leave anytime they wanted to leave. Many Southerners moved to Brazil and became Confederados. Many of their descendants are still down there waving Confederate flags. Many of them still speak some English.
Everyone who lived in the South was an American citizen. As American citizens, each of them had rights under the Constitution. The secessionists tried to deprive those American citizens of their rights under the U.S. Constitution and tried to deprive them of their American citizenship.
Like many of the bigwigs of his day, George Washington was very dissatisfied with the Articles of Confederation because it left the nation weak and indefensible. He pushed for the creation of an indissoluble Union of the States. Our Constitution makes it clear that it was a bond by and between "We the People of the United States" and not just another league of states. In fact, it was for that very important reason that Patrick Henry was opposed to ratifying the Constitution - "Who authorized them to speak the language of, We, the people, instead of, We, the states?"
The secessionists had the right to leave the plantation. They had no right to try to break the constitutional bonds between the American people or to deprive any Americans of their rights under the U.S. Constitution.
No, Fascists always want the people to leave the USA. That is the rub, the USA is a construct of the states-not the other way around. Seceding states need not worry what little Hitlers think.....
You do realize that every state that seceded put secession to the people as referendum, and they OVERWHELMINGLY voted for it? Maybe you don't, you could be that ignorant.