There has been some slight rumbling in modern times of some states wanting to leave the union. Several media personalities have joked-not-joked let them go, we're better off without them.
This got me wondering, was there any serious advocating in the North of just let the south succeeded prior to the American Civil War? No war, just let the slave states form their own independent union? If so, how did the battle of Fort Sumter effect their stance?
That was a matter of great discussion. There certainly was support for letting the south secede in the north. There were significant portions of the north who felt that secession was legal under the 10th Amendment. Throw in the impact of slavery on passing any number of laws, the feeling in a lot of areas was that this would get rid of the south and its problems from the United States. Some areas, such as southern Indiana were very sympathetic to the south.
Those arguments were mostly silenced when the South fired on Fort Sumter. There was still an undercurrent of support in the north with the Copperheads. In states like Maryland and Missouri, southern sympathizers were imprisoned. But, a shooting war and one where they had not fired the first shots was another matter and a large number of southern supporters flipped to supporting the war.
(The firing also led to Virginia, Tennessee, and North Carolina siding with the south).
There were a number of other options being pursued. The Governor Thomas Hicks of Maryland was supportive of breaking the United States into regional Confederacies. He favored a Mid-Atlantic Confederacy with Pennsylvania, New Jersey, and New York. He's presented as being pro-Union in the Civil War, but there's plenty of indications that he was more Anti-Cotton States than Pro-Union. He went with the United States because he had to back one of the two factions and the United States had, by far, the upper hand.
The mayor of New York City, Fernando Wood, actually proposed that New York City secede from the United States. He was anti-war and pro-southern for the entirety of the war and had enough support in New York to be elected to Congress during the war where he opposed the 13th Amendment.
Clement Laird Vallandigham was a pro-southern Congressman from Ohio. Over the course of the war, he was arrested for his position and then expelled out of the United States. (He secretly came back across the border with Canada a short time later. He remained influential in Ohio politics until his death in 1871.
There were many others in other areas. These are just a few examples.