But the fact is there was no "invading army" before Confederates provoked, started & formally declared war on the United States.
Immediately after Fort Sumter, Jefferson Davis began sending military aid to Confederates fighting in Union Missouri.
After Fort Sumter the first Union troops killed were in Union Maryland.
After Fort Sumter the first battles were in Union states like West Virginia, Missouri and even Virginia while it was still officially a Union state.
In all of 1861 of 35 larger battles fought, 25 (71%) were in Union states & territories and nearly 60% of Confederate casualties fell in the Union.
So again, please explain: in what possible way did the 1861 Confederate invasion of the Union not represent an existential threat to the United States.
I think the Navy counts too.
After Fort Sumter the first battles were in Union states like West Virginia,
When was it again that West Virginia became a Union state? And how did it do that since the constitution expressly forbids it?