[Bubba Ho-Tep]

I'm sure that it will make your head explode, but legally (as the United States saw it), the southern states never left the Union. However, their actions in walking out of congress and then fighting a war meant that their delegations wouldn't be accepted back until they had passed constitutions guaranteeing a republican form of government, per Article IV, Sec. 4 of the Constitution. Further, Article I, Sec. 5 says Congress "shall be the Judge of the Elections, Returns and Qualifications of its own Members."

The dates that are given as "readmission" are actually (and go read the actual laws if you don't believe me) readmission of their delegations to Congress.

An Act to admit the State of Texas to Representation in the Congress of the United States. March 30, 1870

[CodeToad]

This will make you head explode but during the time the States were considered having left the union. It wasn’t until many years later that by USSC decree that the court claimed the States had no legal right to leave so they didn’t. Even records of the time consider their actions as re-admission into the union. If the States had not left then many things during the time would have been unconstitutional including Lincoln’s emancipation proclamation. He would have had no legal right to do that, not that the tyrant gave a shit about the law anyway.