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Basically the Kansas-Nebraska Act Let the states decide whether or not they would allow slavery. And it made the northerners mad because it violates the Missouri Compromise. The Missouri Compromise was set to keep slavery out of the northern states. So the Kansas-Nebraska Act screwed the north of pretty much. Let me know if that makes sense lol
Kansas-Nebraska Act (1854)
Made northerners so mad that they created the Republican Party to protest this law. Northerners were mad because residents in Kansas were allowed to use popular sovereignty (they were allowed to vote and decide to allow or stop slavery in Kansas) and this meant the 36-30 line was repealed or erased (remember that the 36-30 line was used in the Missouri Compromise of 1820 and that no slavery was supposed to be allowed in places like Kansas that were above or on the north side of the line).
I could have copy and pasted that too... but i did a little research and put it in my own words so that its understood easier.