What event in 1861 led to the outbreak?

The outbreak of the American Civil War in 1861 was primarily triggered by the Confederate attack on Fort Sumter in South Carolina on April 12, 1861. This confrontation followed the secession of several Southern states from the Union, driven by tensions over slavery and states' rights. The attack on the fort marked the official start of hostilities between the North and the South, leading to a full-scale war.