Which event marked the beginning of the Civil War?

The attack on Fort Sumter marked the beginning of the Civil War because it was the first military engagement of the conflict, occurring on April 12, 1861. This event followed the secession of several Southern states from the Union and their establishment of the Confederate States of America. The Confederate forces' bombardment of Fort Sumter, a Union-held fort in South Carolina, signaled a shift from political tensions and disagreements between the North and South to active military confrontation. This attack rallied support for the war in both the North and the South, solidifying the divide and marking the official start of armed conflict. The implications of this event set the stage for the larger conflict that would ensue, making it historically significant as the point at which the Civil War began.

The other options represent events that occurred either before or after the war had already begun—such as the signing of the Declaration of Independence, which established the United States, the assassination of Abraham Lincoln, which happened in 1865, and the Battle of Antietam, fought in 1862, which was one of the bloodiest battles of the war but did not initiate the conflict itself.