But upon reflection, a good many historians have questioned their explicit assertions that overwhelming numbers and resources made Northern victory inevitable. If that is true, the Confederate leaders who took their people to war in 1861 were guilty of criminal folly or colossal arrogance I'm no Civil War scholar, but I believe they led their people to secede from the Union, not to take up arms against it. It was the North that determined this a causus belli.
Had the South Carolina fire-eaters permitted the US Navy to relieve Fort Sumter, it's very likely that some of the "Northern South" (Virginia, Tennessee, North Carolina) would not have seceded and served as a buffer zone, ensuring the survival of a smaller, but secure Confederacy.
So you have to point to the gunfire on Sumter, 12 April 61, as the precipitate "casus belli"