Hostilities began on April 12, 1861, when Confederate forces fired upon Fort Sumter, a key fort held by Union troops in South Carolina. Lincoln called for every state to provide troops to retake the fort; consequently, four more slave states joined the Confederacy, bringing their total to eleven. Lincoln soon controlled the border states, after arresting state legislators and suspending habeas corpus,[6] ignoring the ruling of the Supreme Court's Chief Justice that such suspension was unconstitutional, and established a naval blockade that crippled the southern economy. The Eastern Theater was inconclusive in 1861 - 62. The autumn 1862 Confederate campaign into Maryland (a Union state) ended with Confederate retreat at the Battle of Antietam, dissuading British intervention. To the west, by summer 1862 the Union destroyed the Confederate river navy, then much of their western armies, and the Union siege of Vicksburg split the Confederacy in two at the Mississippi River. In 1863, Robert E. Lee's Confederate incursion north ended at the Battle of Gettysburg. Lincoln issued the Emancipation Proclamation, which made ending slavery a war goal. Western successes led to Ulysses S. Grant's command of all Union armies in 1864. In the Western Theater, William T. Sherman drove east to capture Atlanta and marched to the sea, destroying Confederate infrastructure along the way. The Union marshaled the resources and manpower to attack the Confederacy from all directions, leading to the protracted Siege of Petersburg. The besieged Confederate army eventually abandoned Richmond, seeking to regroup at Appomattox Court House, though there they found themselves surrounded by union forces. This led to Lee's surrender to Grant on April 9, 1865. All Confederate generals surrendered by that summer.

The American Civil War, widely known in the United States as simply the Civil War as well as other sectional names, was fought from 1861 to 1865. Seven Southern slave states individually declared their secession from the United States and formed the Confederate States of America, known as the "Confederacy" or the "South". They grew to include eleven states, and although they claimed thirteen states and additional western territories, the Confederacy was never recognized by a foreign country. The states that did not declare secession were known as the "Union" or the "North".