Which event is considered as the first conflict that marked the start of the Civil War?

The attack on Fort Sumter is considered the first conflict that marked the start of the Civil War because it was the initial military engagement between the Union and Confederate forces. On April 12, 1861, Confederate troops fired upon Fort Sumter, a Union stronghold located in Charleston Harbor, South Carolina. This attack prompted President Abraham Lincoln to call for 75,000 volunteers to suppress the rebellion, effectively establishing the war's commencement.

The significance of this event lies in its role as the catalyst that transformed the political tensions between the North and South into an armed conflict, serving as a clear signal that negotiation and compromise were no longer viable solutions to their differences. The other events listed, such as the Battle of Bull Run, Battle of Gettysburg, and Siege of Vicksburg, occurred later in the war and represented significant battles rather than the initial outbreak of hostilities.