Lincoln, the most important person at the time the war started (which is when the cause of a war is determined), explicitly stated that it wasn’t about slavery.
That’s a fact.
Sure, Fort Sumter wasn't about slavery, nor was the Confederate Declaration of War on May 6, 1861.
But Declarations of Secession were all about slavery, and within a few weeks of Fort Sumter, slavery became a military issue in the form of "Contraband of War".
Beginning in 1861 Congress passed several laws protecting fugitive Confederate slaves and offering compensated emancipation in Union areas..
The result was slavery became increasingly important, militarily & politically, especially so after Lincoln's preliminary Emancipation Proclamation in 1862.
Those are facts, FRiend.