In response to the election of Abraham Lincoln as President of the United States, 11 southern slave states declared their secession from the United States and formed the Confederate States of America ("the Confederacy"); the other 25 states supported the federal government ("the Union"). After four years of warfare, mostly within the Southern states, the Confederacy surrendered and slavery was outlawed everywhere in the nation. Issues that led to war were partially resolved in the Reconstruction Era that followed, though others remained unresolved.
Hostilities began on April 12, 1861, when Confederate forces attacked a U.S. military installation at Fort Sumter in South Carolina. Lincoln responded by calling for a volunteer army from each state to recapture federal property, which led to declarations of secession by four more slave states. Both sides raised armies as the Union seized control of the border states early in the war and established a naval blockade. Land warfare in the East was inconclusive in 1861–62, as the Confederacy beat back Union efforts to capture its capital, Richmond, Virginia, notably during the Peninsular Campaign. In September 1862, the confederate campaign in Maryland ended in defeat at the Battle of Antietam, which dissuaded the British from intervening. Days after that battle, Lincoln issued the Emancipation Proclamation, which made ending slavery a war goal.
Slavery for the Confederacy's 3.5 million blacks effectively ended when Union armies arrived; they were nearly all freed by the Emancipation Proclamation. Slaves in the border states and those located in some former Confederate territory occupied prior to the Emancipation Proclamation were freed by state action or (on December 18, 1865) by the Thirteenth Amendment. The full restoration of the Union was the work of a highly contentious postwar era known as Reconstruction.
The war produced about 1,030,000 casualties (3% of the population), including about 620,000 soldier deaths—two-thirds by disease.
One reason for the high number of battle deaths during the war was the use of Napoleonic tactics, such as charging. With the advent of more accurate rifled barrels, Minié balls and (near the end of the war for the Union army) repeating firearms such as the Spencer repeating rifle, soldiers were mowed down when standing in lines in the open. This led to the adoption of trench warfare, a style of fighting that defined the better part of World War I.
The war destroyed much of the wealth that had existed in the South. Income per person in the South dropped to less than 40% than that of the North, a condition which lasted until well into the 20th century. Southern influence in the US federal government, previously considerable, was greatly diminished until the latter half of the 20th century