Deport, arrest, or fine secessionists of the United States.

Secession is essentially ILLEGAL
In Article 6 Section 3 of the Constitution it clearly states that "the states shall be bound by oath or affirmation to support this Constitution." They agreed to be part of the Union forever. In Article 1 Section 10 of the Constitution it says "No state shall enter into any treaty, alliance, or Confederation." In order to secede from the Union and form their own country, they needed to make treaties and alliances with other states. In Article 4 Section 3, it states that "Congress will make rules and regulations for territories and for property of the federal government." The state or territory on its own cannot create the rules for itself. If the South secedes, they are taking the property of the federal government without getting permission from Congress.