In the 1860 presidential election, Republicans, led by Abraham Lincoln, opposed expanding slavery into United States' territories. Lincoln won, but before his inauguration on March 4, 1861, seven slave states with cotton-based economies formed the Confederacy. Outgoing Democratic President James Buchanan and the incoming Republicans rejected secession as illegal. Lincoln's inaugural address declared his administration would not initiate civil war. Eight remaining slave states continued to reject calls for secession. Confederate forces seized numerous federal forts within territory claimed by the Confederacy. A Peace Conference failed to find a compromise, and both sides prepared for war. The Confederates assumed that European countries were so dependent on "King Cotton" that they would intervene; none did and none recognized the new Confederate States of America.
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 each state to provide troops to retake the fort; consequently, four more slave states joined the Confederacy, bringing their total to eleven. The Union soon controlled the border states 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. Lincoln issued the Emancipation Proclamation, which made ending slavery a war goal. 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. Western successes led to Ulysses S. Grant 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, and could afford to fight battles of attrition through the Overland Campaign towards Richmond, the Confederate capital. The defending Confederate army failed, leading to Lee's surrender to Grant at Appomattox Court House on April 9, 1865.
The American Civil War was one of the earliest true industrial wars. Railroads, the telegraph, steamships, and mass-produced weapons were employed extensively. The mobilization of civilian factories, mines, shipyards, banks, transportation and food supplies all foreshadowed World War I. It remains the deadliest war in American history, resulting in the deaths of an estimated 750,000 soldiers and an undetermined number of civilian casualties.