[SCDogPapa]

The minute they do so, they are a foreign country with a foreign government (who fires the first shot, even!), and then the US Congress can legally declare war on them - occupy the land - and bring those states back in once the war is won.

If I'm not mistaken,,Congress did not declare war until 3 months after Lincoln had ordered the invasion of the South.

[Grand Old Partisan]

You are mistaken. Congress never declared war, which is an act taken against a foreign country not a domstic rebellion as was the Confederacy.

[Non-Sequitur]

If I'm not mistaken,,Congress did not declare war until 3 months after Lincoln had ordered the invasion of the South.

You're mistaken. The confederate congress passed a resolution declaring a state of war existed on May 6, 1861. Neither President Lincoln or the U.S. Congress ever issued a declaration of war. Such a declaration would be inappropriate. You declare war on other sovereign nations, not on rebellious sections of your own.