Slavery

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Former Slaves
As America expanded after the Louisiana Purchase and Mexican War, strong debate began over which new states would be slave or slave-free states. The Missouri Compromise passed in 1820 made a rule that prohibited slavery in states from the Louisiana Purchase. During the Mexican War, conflict started about what would happen with the new states the U.S. expected to gain. David Wilmot proposed the Wilmot Proviso in 1846 which would ban slavery in the new states, however, this idea was shot down after much debate. The Compromise of 1850 was created by Henry Clay to deal with the balance between slave and non-slave states. 
Slavery is the most supported reason for the war and almost everybody thinks that slavery was the only reason for the war. Many others say that the Union might have misunderstood the Confederates about slavery.

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Old Slave Quarters