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The Civil War was caused by sectionalism, slavery, and states' rights tensions. The Northern and Southern states disagreed on economic, social, and political issues. Slavery was a major point of contention, with the South relying on it economically and the North opposing it morally. States' rights played a role as well, as the South believed they had the right to determine their own laws, including slavery. When Lincoln was elected president and threatened their way of life, Southern states seceded based on states' rights and the desire to protect slavery. This marked the start of the Civil War.