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Missouri Compromise
The Missouri Compromise made it that all new states above the longitude 30'36 would be a free state, and anything below would be a slave state. This could upset both the north and the south because there could be more free states than slave states, or more slave states than free states -
Dred Scott Case
Dred Scott was a slave that live in a free state, but he thought that because he was in a free state, he should be free as well, so he decided to attempt to sue his master and win his freedom. When his case got to the court, the court said that any slave moved in to a free state by an owner, is still a slave. This angered the North because they made the free states to remove slavery from those states, this goes against the whole idea of having free states in the first place.