What is the main reason the us civil war started?

While many still debate the causes of the Civil War, author James McPherson said that, "The Civil War started because of uncompromising differences between the free and slave states over the power of the national government to prohibit slavery in the territories that had not yet become states. When Abraham Lincoln won the election in 1860 as the first Republican president on a platform pledging to keep slavery out of the territories, seven slave states in the deep South seceded and formed a new nation, the Confederate States of America. The incoming Lincoln administration and most of the Northern people refused to recognize the legitimacy of secession. They feared that it would discredit democracy and create a fatal precedent that would eventually fragment the no-longer United States into several small, squabbling countries."

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pennsylvania, new hampshire, massachusetts, connecticut and rhode island .