Originally Posted by
Herod
Lonewolf will correct me here but, the land that is occupied by the American States already belonged to the USA, which had claimed "from sea to shining sea". The States merely gained statehood as they became viable.
That was after the USA was formed from the original 13 states, and various associated territories. States formed and then applied (see California as a good example) for entry into the Union. (I seem to recall that California did send some years as a republic?).
The secession of the south was illegal, because they were taking possession of something that had never belonged to them; i.e. the land of the United States.
Not going to digress into that, as that particular point is argued from a variety of points of view on Constitutional grounds. From the PoV of the government in Washington at the time, it was a rebellion and there was the serious problem of confiscating federal property... among other things. That's best left for a different discussion than this one.