What event triggered the Civil War?

The attack on Fort Sumter in 1861 is widely recognized as the event that triggered the Civil War because it marked the first military confrontation between the Union and Confederate forces. On April 12, 1861, Confederate troops opened fire on Fort Sumter, a Union-held fort in South Carolina, which had been a flashpoint following states' secession from the Union. This assault led President Abraham Lincoln to call for troops to suppress the rebellion, effectively igniting the full-scale conflict that would become the Civil War.

While the election of Abraham Lincoln heightened tensions and contributed to the secession of Southern states, it was the conflict at Fort Sumter that resulted in the actual commencement of hostilities. Similarly, John Brown's raid at Harpers Ferry and the passage of the Kansas-Nebraska Act significantly increased sectional tensions, but they did not directly lead to the outbreak of military action. Instead, they are part of the broader context of conflict over slavery and states' rights leading up to the war. The attack on Fort Sumter is explicitly considered the moment that transitioned political and social tensions into active warfare.