The U.S. Civil War was prompted by several interconnected factors, primarily the issue of slavery and its expansion into new territories. The debate over whether states should have the right to allow slavery fueled regional tensions between the North and South. Economic disparities, differing social structures, and states' rights further exacerbated these divides. Events like the Kansas-Nebraska Act and the Dred Scott decision heightened polarization, leading to increased sectionalism. Ultimately, the election of Abraham Lincoln in 1860 triggered the secession of Southern states, culminating in the outbreak of war.