[DiogenesLamp]

The day a recognized sovereign nation attacked the United States of America.

For the purpose of creating a fictional legal justification for invading states that sought independence, Lincoln asserted they never left the USA.

For the purpose of denying them their constitutional rights after they had been beaten by the 4 times larger manpower of the Northern states, they were regarded as having "left" the United States.

Their status existed in a quantum state of superposition, being both "in" the Union and "out" of the Union simultaneously, depending upon what abuse of power needed to be justified at any given time.

[BroJoeK]

DiognesLamp: "For the purpose of denying them their constitutional rights after they had been beaten by the 4 times larger manpower of the Northern states, they were regarded as having "left" the United States."

Nonsense, but former Confederate states were regarded, factually, as having left Congress and so Congress set certain conditions for their return.
And since most post-war Southern states were then dominated by Republican voters, they were happy to comply.

That willing compliance lasted until roughly 1876, when Reconstruction ended and Southern Democrats again seized power in former Confederate states, imposing Black Codes, segregation and voting restrictions on non-Democrats for most of the next 100 years.