Why did the Battle of Fort Sumter lead to a wider war?

The Battle of Fort Sumter served as the catalyst for a wider war by forcing a definitive choice between the Union and the Confederacy, effectively ending the period of uneasy hesitation that followed the initial secession of seven Southern states.12

When Confederate forces opened fire on the fort on April 12, 1861, it transformed a political standoff into an active military conflict. For President Abraham Lincoln, the attack provided a clear casus belli, allowing him to frame the response as a defense of the Union and the government's authority. Following the surrender of the fort, Lincoln issued a call for 75,000 militiamen to suppress the rebellion, a move that compelled the states of the Upper South—Virginia, Arkansas, Tennessee, and North Carolina—to choose sides, ultimately leading them to secede and join the Confederacy.132

This escalation solidified the polarization between the North and South, turning a regional political crisis into the American Civil War, the deadliest conflict in United States history.13