You must not have read previous Civil War threads, or you'd well understand by now that Civil War did not start over either secession or slavery.
Nor did it start over any of the other reasons often listed by pro-Confederates: tariffs, trade restrictions, "haughty northerners" or even John Brown's raid.
None of that.
Civil War only began after months of Confederate provocations, in seizing dozens of major Federal properties -- forts, ships, arsenals, mints, etc -- threats against Union officials & firings on Union ships.
Civil War only began after the Confederacy launched a military assault on Union troops in Union Fort Sumter (April 12, 1861) and the Confederate Congress formally declared war on the United States (May 6, 1861), sending military aid to pro-Confederates in Union Missouri.
All of that happened before a single Confederate soldier was killed in battle with any Union force and before any Union army invaded a single Confederate state.
The Confederate assault on Fort Sumter served the same purpose as the Japanese attack on Pearl Harbor, December 7, 1941: an unequivocal act of war which united the United States in response to a fully recognized existential threat.
Oh, I know all that and more. My wife was a history major and taught high school for ten years. I could go on and on.