The story most Americans are given of the origins of the Civil War are fairly consistent, though perhaps different sides of the same story depending on where in the US you went to school. What may surprise some people is to see where the idea of states rights and secession came from. When I was in grade school my understanding was the right of secession was a difference in opinion on the structure and purpose of the unification of the states.
In reality, the idea of breaking off from the US was a very real possibility early in the 19th century, and the leader of the idea will likely surprise you quite a bit.
First, we need to talk about the political parties of the beginning of the 19th century. There were the Federalists and the Democratic-Republicans. Broadly speaking, the Federalists were nationalists who believed in control by the federal government and distrusted democracy. Democratic-Republicans, also called Republicans or Jeffersonian Republicans. Later on it split into two parties, the Democrats and the Republicans. They believed strongly in states rights.
In 1798 President John Adams, a Federalist, with the help of a friendly Congress, passed the Alien & Sedition Acts. Among other things, it gave the President the right to deport or arrest non citizens, and it also allowed the suppression of free speech. It was very unpopular with most immigrants, and it helped get Thomas Jefferson into the White House in the next election.
Prior to John Adams being voted out of office, the Alien & Sedition act caused quite a stir. Then Vice President Thomas Jefferson and James Madison secretly wrote the Kentucky & Virginia Resolutions and they were passed by the states legislatures. Kentucky’s, written by Thomas Jefferson, the man so famous for writing the Declaration of Independence, took states rights to the extent that a state could not only oppose federal decision, but secede from the union. George Washington and James Garfield are famous for their responses to Jefferson’s resolution, identifying it as a being able to dissolve the union, and one of the causes of the Civil War.
So the justification for the secession of the southern states was not something imagined to justify opposition to federal decisions or presidential positions they didn’t agree with – it was the continuation of the ideals of one of the founding fathers.