The day a recognized sovereign nation attacked the United States of America.
Poor choice.
Well... the day a pretend but internationally unrecognized conspiracy of "...combinations too powerful to be suppressed by the ordinary course of judicial proceedings..." attacked the United States, thus triggering Pres. Jefferson's 1807 Insurrection Act (which replaced Pres. Washington's 1792 "Calling Forth Act") for the call-up of Federal troops.
But yes, all things considered, it was a poor choice.
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.
Recognized by whom?