Because they weren't defending. They were invading. The Sixth Massachusetts opened fire on citizens of Maryland, and Abraham Lincoln ordered the United States Navy and the Militias of the Northern States to wage war on the seceding States, to subjugate them.
The Northern States would have been entitled to use deadly force to defend their interests if Beauregard had been ordered north to seize and fortify Sandy Hook and Staten Island in an attempt to extract revenue from the Atlantic trade in New York harbor.
They invaded because the federal gov't was attacked.
The Consititution states that no state has a right to form a confederation with another state or states.
Section 10. No state shall enter into any treaty, alliance, or confederation;
The Confederate Constitition said the same thing.