During Buchanan's presidency, tensions between the North and the South over slavery and states' rights intensified, leading to the secession of several Southern states and eventually the outbreak of the Civil War in 1861. Buchanan did not take any significant action to defuse the situation or to prevent war. Instead, he supported the pro-Southern policies of his cabinet members, who attempted to extend slavery into the territories and to protect the rights of slave owners.
Buchanan's failure to act decisively or to find a compromise between the North and the South is widely seen as a major mistake that contributed to the outbreak of the Civil War and the profound damage it caused to the United States.