4. Start of the Civil War (1861)

On April 12, 1861, Confederate forces fired on Fort Sumter in Charleston Harbor, South Carolina, marking the outbreak of the American Civil War after years of escalating tensions over slavery, states’ rights, and economic differences.

The attack prompted President Abraham Lincoln to call for volunteers, leading 11 Southern states to secede and form the Confederacy.
The four-year conflict became the deadliest in U.S. history, claiming over 600,000 lives and ultimately preserving the Union while ending slavery.