“President Lincoln refused to surrender to Confederate military aggression against the United States and its Constitution.”
That must have been the famous Confederate invasion of the North back in July 1861.
If not for the valiant defensive forces raised by Lincoln the United States above the Mason Dixon line would surely have been conquered by the huge Confederate army poised for its conquest. You, BroJoeK, have the historical situation of the time exactly right.
The United States July 1861 "invasion" of Virginia didn't happen until after Confederates had illegally seized dozens of Federal facilities -- forts, ships, arsenals, armories, mints, etc. -- threatened and fired on US officials (January through April, 1861), and threatened & launched a military assault on Federal troops in the Federal Fort Sumter (April 12, 1861).
Indeed, it didn't happen until after Confederates:
Pelham: "If not for the valiant defensive forces raised by Lincoln the United States above the Mason Dixon line..."
On April 12, 1861 the United States included six states South of the Mason Dixon line: Maryland, Virginia, North Carolina, Kentucky, Tennessee and Arkansas.
After Confederates assaulted Fort Sumter, four of those states declared secession, and Confederates soon sent forces to invade the others -- plus West Virginia which had declared its secession from the secessionists.
Missouri was North of the old Mason-Dixon, but that didn't stop Confederates from attempting to seize it from the Union by military force.
So, Confederate assault on Fort Sumter was the Confederacy's Pearl Harbor, and Bull Run / Manassas equivalent to some early WWII battle -- Kasserine Pass comes to mind.
No, that would be the famous bombardment of Fort Sumter in April 1861.