|Previous: Mexican-American War, Raid on Harpers Ferry|
|Concurrent: Colorado War|
|Next: Reconstruction, Plains Indian Wars|
|American Civil War|
|Date: 12 April 1861-9 May 1865|
|Place: Southern and western United States|
| Outcome: Union victory:|
The American Civil War began as a result of the secession of the Confederate States from the union in response to the election of President Abraham Lincoln, a member of the Republican Party and a famous abolitionist. The southern states had been talking of secession since the 1830s under John C. Calhoun, as the South felt that the federal government was impeding on its people's rights by pressing for the abolition of slavery and by supposedly prioritizing the affairs of the North over those of the South. On 20 December 1860, South Carolina became the first state to secede from the union, followed by Mississippi, Florida, Alabama, Georgia, and Louisiana by 26 January 1861. Several US Army soldiers in these states defected to the Confederacy or were forced to surrender at their outposts, and many American generals from the South resigned their commissions to join the newly-formed Confederate States Army. On 8 February 1861, the Confederacy passed a provisional constitution, and Democratic Party politician Jefferson Davis was elected President. Most Union forts in the seceding states were convinced to surrender peacefully, but this came to an end on 12 April 1865, when P.G.T. Beauregard's Confederate army laid siege to Fort Sumter in South Carolina. The Union garrison refused to surrender, leading to the bombardment of the fort. Although the only death in the battle was the result of an accidental gunpowder explosion within the fort, the Union took the Confederate attack as a declaration of war, and President Lincoln called up 75,000 volunteers on 15 April. Many slave states refused to take up arms against their neighbors, and from 17 April to 20 May Virginia, Arkansas, Tennessee, and North Carolina also seceded from the Union to join the Confederacy. The Confederacy's new capital was the Virginia state capital of Richmond, while the Union capital remained at Washington DC. For four years, the Confederacy and Union would engage in a series of bloody battles that turned neighbors against neighbors, with the Confederacy winning most of the battles of 1861 before the Union began to recover in 1862-1863 and launch a series of decisive counterattacks in 1864 and 1865.
The war was the first war to see the use of machine guns and ironclads, both of which shaped land and naval warfare forever. Most armies still engaged in the traditional style of battle, involving the use of firing lines by regiments, but they were equipped with new weapons, including Colt and Henry repeater rifles. Warfare was brutal at times, with the Union waging a war of attrition on the South by 1864. William T. Sherman introduced the concept of "total war" with his March to the Sea, ravaging the states of Georgia and South Carolina during his march from Tennessee up the Atlantic Ocean coast. He figured that civilians were a part of the war as well, and destroying railroad lines, towns, public works, and farms would be crucial to winning the war, as it drained the Confederacy of supplies and morale. The war saw former West Point classmates fight against each other in battle, many of whom had fought together in the Mexican-American War, and it was common to see former neighbors, friends, and even family members fight against each other. The war would come to an end in April 1865 when the Union captured the Confederate capital of Richmond after the successful Overland Campaign and the Siege of Petersburg, and the Confederate general Robert E. Lee surrendered the main Confederate army at Appomattox Court House on 9 April 1865. President Abraham Lincoln was assassinated by a Confederate sympathizer while viewing a play at Ford Theater on 14 April 1865, but the war was already coming to an end. On 2 May 1865, President Andrew Johnson proclaimed that the war had met its end, and that the union was preserved. 647,000 Union soldiers and 427,000 Confederate soldiers were killed or wounded in the war, making it the bloodiest war in US history; the bloodiest single-day battle in US history was the Battle of Antietam in 1862. The war was followed with social change, as Lincoln's Emancipation Proclamation freed the slaves, and the US Republican Party became the dominant party in the north. The Civil War's political and social divisions are still felt today, as conservatism and states rights are still major beliefs in the south, while liberalism and multiculturalism are major beliefs in the north.