IIRC, the secessions were preceeded by a period where it was sort of an open secret of what’s gonna happen, with stuff like future confederate generals transferring cannon stock to southern bases, and moving themselves and their families south, and the North by and large stood by and let it happen under the rationale that “we shouldn’t anger them, they may still come around”. To the point that, again, IIRC, the commander of Fort Sumter saw trouble brewing across the river and requested reinforcements just before the war, and was turned down.