In my opinion the document which lead irreversibly to the War of Northern Aggression was the 1820 Missouri Compromise. This compromise which mandated the admission of state in pairs, one a slave state and one free, made it almost impossible for any amendment abolishing the slavery clause from the Constitution to reach the 2/3 majority needed to amend the Constitution. there would always be a 50/50 split in Congress and the states legislatures.
Because of this, the slavery Clause stood unchanged. States entering the Union were denied the right to choose for themselves. And the decision as to whether a state came into the Union was made on the basis of geography and NOT by a choice of the people. In 1857 in the Dred Scott ruling the Missouri Compromises of 1820 and of 1850, the Wilmot Proviso, as well as the Northwest Ordinance were rendered UNConstitutional by the United States Supreme Court which ruled correctly that the provision within the Constitution which embodied the Institution of slavery only gave that power to regulate the practise to the individual States and not to the federal Goverment. That what was legally held property in ANY state could not be denied to the owner of that property because he carried that property into a state that did not allow slavery.
We still practise that same principal today in that a Car that is registered in New York can still be driven legally in California without it having to be reregistered so long as the owner does not take up legal residence in California.
The Dred Scott Ruling is what started the War. Because it reversed all the efforts of the abolishionist to that point, and open the door where states, that already were non-slave state in the Union, COULD reestablish the practise within their state if they wished. The abolishionist could not allow that ruling on the Constitutionality of slavery and States Rights to stand uncontested.
IF the 1820 Missouri Compromise not been enacted would the States have decided differently than they did on whether or not they would allow or disallow slavery within their states as they entered the Union? Would there have been a balance of power shift that could have changed the Constitution by legal process to eliminate slavery? Or not eliminate it? Instead of the 50/50 mess that lead us into being divided into the North and the South.