Simple question: If the states were sovereign, why did the Union invade?
They weren't. Show me where in the Constitution it says they were. I see lots of stuff telling what the states can't do--coin money, enter into treaties, keep troops in time of peace, etc. (Article 1, Section 10)--all of which doesn't sound very sovereign to me. Article 4, Section 4 also gives the federal government power over state governments by ensuring their nature. They were invaded because they were in rebellion. They were invaded because they fired on U.S. troops at Ft. Sumter.
Now you answer my question.