Which they saw as an infringement upon their rights under the constitution that they hadn't agreed to. The constitution was clearly designed to protect slavery, and if you read the debates about slavery during the convention, it is clear that the non slave states (which were an extreme Minority in 1787) accepted that the nation would be a slave owning nation. The Slave owning states made it clear in the convention that if the institution of slavery was not accommodated, they would refuse to ratify the new Constitution.
Nobody at the time suggested that slaves would be banned from the territory. From the gist of the discussion it is clear that the Slave owning states would have never agreed to that.
But saying the war wasn't about slavery is silly. The states that left said WHY they left.
A few of them said they left because of slavery, but the vast bulk of them did not say that. Virginia was perhaps the most important of all the states of the Confederacy, go look in their secession statement and see if it says anything about leaving over slavery.
People keep repeating that the Confederate states left over slavery, even though it would have remained legal in the Union into the foreseeable future, but they repeat this because this is what they truly wish to believe, because in their minds it justifies all the bloodshed and the very bad thing they did to the people of the South.
It is the only way in their mind they can justify all the unnecessary murder of people who just wanted to be left alone by Washington DC.
So...the South didn’t leave over slavery, but...what? The US government has grown immensely and excessively, but just HOW had it become an enormous monster in the 70 years since the Constitution had been adopted?
Seems to me the problem was that the slave owning states no longer were represented in parity with the non-slave states. Their population meant they were a minority in the House, and with the admission of Minnesota, they became a minority in the Senate and in the Electoral College. And would become more so, with each new non-slave state.
There is only one sentence in the Virginia statement that discusses the reason they left: “ and the Federal Government having perverted said powers, not only to the injury of the people of Virginia, but to the oppression of the Southern slaveholding States.”
“the oppression of the Southern slaveholding States”
That is the ONLY reason given by Virginia. Not that taxes were being passed without representation, but that the majority of the country was oppressing the minority of SLAVEHOLDING STATES.
Yet Slavery was not the problem?