The American Civil War: Map of the Union and the Confederacy

By 1860 the United States was split between free states in the North and fifteen slave states in the South, where about four million people were held in slavery. For decades the two sections had fought over whether slavery could spread into the new western territories, through the Missouri Compromise of 1820, the Compromise of 1850, the violence of "Bleeding Kansas" and the Dred Scott decision of 1857. The new Republican Party was pledged to stop slavery from expanding.

When the Republican Abraham Lincoln won the presidential election of November 1860 without carrying a single Southern state, South Carolina seceded on 20 December 1860. Mississippi, Florida, Alabama, Georgia, Louisiana and Texas followed, and in February 1861 they formed the Confederate States of America at Montgomery, Alabama, with Jefferson Davis as president. Their declarations of secession made the defence of slavery their central reason.

On 12 April 1861 Confederate batteries opened fire on Fort Sumter, a federal fort in the harbour of Charleston, South Carolina, which surrendered the next day. Lincoln called for 75,000 volunteers to put down the rebellion, and four more states, Virginia, Arkansas, North Carolina and Tennessee, joined the Confederacy rather than fight against it. Richmond, Virginia, became the Confederate capital, only about 160 km from Washington.

The slave states of Delaware, Maryland, Kentucky and Missouri stayed in the Union, as did the western counties of Virginia, which became the state of West Virginia in 1863. Holding these border states, with their rivers, railways and men, was vital to Lincoln. Press 1861 on the map to see the divided country.

Both sides expected a short war. Instead, the Confederate victory at the First Battle of Bull Run, near Washington, on 21 July 1861 showed it would be long. The Union adopted General Winfield Scott's "Anaconda Plan": blockade the Southern coast, the dashed line on the map, and seize the Mississippi to split the Confederacy. In the West, Ulysses S. Grant took Forts Henry and Donelson in February 1862 and won the bloody Battle of Shiloh in April, and Admiral Farragut captured New Orleans, the South's largest city, at the end of April 1862.

In the East, Robert E. Lee took command of the Army of Northern Virginia in June 1862, drove the Union army back from Richmond in the Seven Days Battles and won again at Second Bull Run. When he invaded Maryland, the two armies fought at Antietam Creek on 17 September 1862, the bloodiest single day in American history, with about 23,000 men killed, wounded or missing. Lee retreated, and Lincoln used the moment to issue the preliminary Emancipation Proclamation.

The Emancipation Proclamation, which took effect on 1 January 1863, declared free all enslaved people in the areas still in rebellion, about 3.5 million people. It could only be enforced where Union armies went, but it made the end of slavery a Union war aim, discouraged Britain and France from recognising the Confederacy and opened the army to Black soldiers. By 1865 about 180,000 African Americans had served in the United States Colored Troops. Many enslaved people freed themselves by escaping to Union lines, as in the drawing in the gallery below.

After a brilliant victory at Chancellorsville in May 1863, Lee invaded the North again. From 1 to 3 July 1863 his army fought the Union Army of the Potomac under George Meade around the town of Gettysburg, Pennsylvania. The battle ended with the failure of Pickett's Charge against the Union centre on Cemetery Ridge; with about 50,000 casualties it was the largest battle of the war, and Lee never invaded the North again. On 19 November Lincoln dedicated a cemetery there with his Gettysburg Address.

On 4 July 1863, the day after Gettysburg, the river fortress of Vicksburg, Mississippi, surrendered to Grant after a six-week siege. With Port Hudson's fall days later, the Union controlled the whole Mississippi and cut off Texas, Arkansas and Louisiana from the rest of the Confederacy. Compare 1862 and 1864 on the map to see the Confederacy shrink.

In March 1864 Lincoln made Grant commander of all Union armies. Grant fought Lee in a brutal campaign through Virginia, from the Wilderness to Cold Harbor, then besieged Petersburg and Richmond for nine months. William T. Sherman captured Atlanta on 2 September 1864, helping Lincoln win re-election in November, and then marched 60,000 men across Georgia to Savannah, destroying railways, factories and crops in a swath up to 100 km wide: the March to the Sea, shown in blue. In 1865 he turned north through the Carolinas.

Lee abandoned Petersburg and Richmond on 2–3 April 1865 and was cornered at Appomattox Court House, where he surrendered to Grant on 9 April. Five days later, on 14 April, the actor John Wilkes Booth shot Lincoln at Ford's Theatre in Washington; he died the next morning. Joseph E. Johnston surrendered the last large Confederate army to Sherman at Bennett Place on 26 April, and the last forces west of the Mississippi gave up on 26 May 1865.

The Civil War was the deadliest war in American history. For over a century the death toll was put at about 620,000 soldiers; a 2011 study of census records raised the estimate to around 750,000, and perhaps more, two-thirds of them from disease. Much of the South lay in ruins. The Union was preserved, and the idea that a state could leave it was settled by force.

The Thirteenth Amendment, ratified in December 1865, abolished slavery throughout the United States, and the Fourteenth (1868) and Fifteenth (1870) Amendments promised citizenship, equal protection and the vote to freed men. During Reconstruction (1865–1877) Black Southerners voted and held office, but after federal troops left, white Southern governments imposed segregation and took away their votes for nearly a century.