Which Crisis Brought the Union Closest to Dissolution

In "The Missouri Controversy and Sectionalism," found in Congress and the Emergence of Sectionalism, Robert Pierce Forbes opines that the Missouri crisis brought "the Union closer to dissolution than at any time prior to the firing on Fort Sumter."

That wouldn't be my choice, but it inspired me to put together a poll on the subject. I'd be delighted if you cast your vote, and even more pleased if you posted a comment explaining your selection.