One of the problems of this period was slavery. By 1861, over three million African-Americans were enslaved in the South. This means that they worked for other people, but had no freedom and received no money for their work. Most worked picking cotton on large plantations. Cotton became the main crop in the South after Eli Whitney invented the cotton gin in 1793. There were a few slave rebellions against slavery, including one led by Nat Turner. All of these rebellions failed. The South wanted to keep slavery, but by the time of the Civil War, many people in the North wanted to end it. Another argument between the North and South was about the role of government. The South wanted stronger state governments, but the North wanted a stronger central government.