“It was the extension of slavery into Federal territories and the violation of free state statutes that precipitated the Confederate attack on the Union which began the war.”
Nonsense, it was the refusal of the Union to remove its troops from Confederate territory, and the attempt by the Union to resupply those troops in defiance of the Confederacy that precipitated the attack. The South had no interest in what happened with regards to slavery in Federal territories by the time the war began, because they had already left the Union.
There was no "Confederate territory."
As the Constitution clearly declares, all state acts - and that includes "Ordinances of Secession" - are subject to Federal review.
No state can unilaterally alienate territory.
Japan claims that it was the U.S. trade embargo in 1940 that placed them in an untenable position and forced them to attack Pearl Harbor. Would you agree with that?
As has been noted here frequently, Communist Cuba also claims that US forces in Guantanamo Bay are illegitimate and must be removed.
But our guys remain there, and any Cuban military assault on the base will be, correctly, seen as an act war against the United States.
The United States tolerated British forts on US territory for 30+ years, before that issue was resolved peacefully in 1814.
Bottom line: regardless of how legitimate, or illegitimate, Confederate claims to Fort Sumter were, their military assault on Union troops there was as clearly an act of war as, for example, the Japanese attack on Pearl Harbor -- a day of infamy.