The same thing that officially got Abe Lincoln shooting Southward would figure here: walking off with Federal property and initiating their own shooting.
It wasn't federal property, it was property held as surety by one State to ensure the performance of the other States to the compact.
When the northern States failed to perform, the Notice of Secession caused the property to revert to the States that initially held it.
In actuality, it was being unlawfully occupied by federal forces, and that's why it was fired upon.
Neither of these rules has been adopted. Each State in ratifying the Constitution, is considered as a sovereign body independent of all others, and only to be bound by its own voluntary act. In this relation then the new Constitution will, if established, be a federal and not a national Constitution.
Federalist, no. 39James Madison, 16 Jan. 1788