The event that marked the start of the U.S. Civil War was the attack on Fort Sumter in South Carolina on April 12, 1861. This confrontation occurred after several Southern states seceded from the Union following the election of Abraham Lincoln in 1860. The Confederate forces fired upon the fort, which was held by Union troops, signaling the beginning of armed conflict. The attack galvanized public opinion in the North, leading to a mobilization of troops and resources to preserve the Union, ultimately resulting in a four-year war that would profoundly reshape American society.