The American Civil War (1861 to 1865) was the pivotal event in United States history and the largest armed conflict in the Western world following the end of the Napoleonic Wars (1815) and prior to the beginning of the First World War (1914). The central cause of the war was the institution of slavery, which had increasingly caused conflict between Southern states, which relied heavily on slave labor for their agrarian economy, and Northern states, which were heavily industrialized and had far less need for slaves.

The post-Civil War claim that the conflict was fought over states' rights, still repeated today, is untenable unless one amends the argument to read, "the Civil War was fought over states' rights to uphold the institution of slavery." Documents from slave-owning states giving their reason for secession from the Union in 1860 and 1861 repeatedly cite the need to uphold slavery, and earlier records and events support the claim that slavery was the foundational cause for all the other problems that had come to divide the North and South in the first half of the 19th century.

The war raged from 12 April 1861, when Confederate forces fired on Fort Sumter, to 9 April 1865, when Confederate general Robert E. Lee surrendered to Union general Ulysses S. Grant at Appomattox Court House. Hostilities continued after April, however, and the final end of the war is often given as 26 May 1865, when Confederate Lt. General Simon B. Buckner surrendered the Trans-Mississippi Department.

The war ended slavery in the United States, abolished by the Thirteenth Amendment, destroyed the plantation system and agrarian economy of the South, further industrialized the North, and cost upwards of 650,000 lives. The Reconstruction Era (1865 to 1877) brought the warring states back into a cohesive Union, establishing the United States of America as it has been commonly understood since.

As noted, the American Civil War was fought over slavery even though, initially, President Abraham Lincoln was only interested in preserving the Union and preventing the spread of slavery into territories that were not yet states. All the major events leading up to the war had to do with disputes between free states and slave states, and these include:

Abolition of the Transatlantic Slave Trade in 1808 cut off the supply of slaves from outside the United States, although the demand by Southern states was high, resulting in illegal smuggling operations in the South and attempts to stop them by the federal government.

The Missouri Compromise of 1820 admitted Missouri as a slave state and Maine as a free state, continuing the balancing act of the government to placate Northern and Southern interests regarding slavery.

Nat Turner's Rebellion of 1831: The deadliest slave uprising in US history, Turner's rebellion encouraged widespread discussion of emancipation while also leading to harsher slave laws that were condemned by Northern abolitionists. Southern slaveholders blamed the abolitionists for the insurrection.

The Nullification Crisis of 1832: South Carolina challenged federal tariffs and asserted states' rights to nullify federal laws and regulations, establishing a foundation later used to justify secession.

Publication of Frederick Douglass' Autobiography in 1845: Narrative of the Life of Frederick Douglass became the bestselling slave narrative prior to the publication of Uncle Tom's Cabin by Harriet Beecher Stowe in 1852. Both works, as well as others by former slaves, increased the momentum of the abolitionist movement.

The Fugitive Slave Act of 1850 mandated that citizens of all states must aid in capturing and returning escaped slaves under threat of fine and/or imprisonment. The law was highly unpopular in the North, causing resentment toward Southern slaveholders.

The Compromise of 1850 established the concept of popular sovereignty in territories, allowing the people to choose whether to become a slave or free state. The law admitted California as a free state but, at the same time, strengthened the Fugitive Slave Act.

The Kansas-Nebraska Act of 1854 allowed popular sovereignty to decide whether the territories of Kansas and Nebraska would be free or slave states, leading to the violence of "Bleeding Kansas" in which pro-slavery and free-staters fought each other, often considered a "dress rehearsal for the Civil War."

The Dred Scott Decision of 1857 ruled that Black people in America were not citizens, had no rights, and so could not bring lawsuits. It also ruled that Congress had no power to ban slavery in territories not yet part of the United States.

John Brown's Raid on Harpers Ferry in 1859: Abolitionist John Brown tried to incite a large-scale slave rebellion in Harpers Ferry, Virginia, in hopes of ending slavery in the United States.

Election of Abraham Lincoln as President in 1860: Lincoln's platform of controlling the spread of slavery westward alarmed Southerners, who saw this as a threat to their way of life. After Lincoln was elected, South Carolina was the first state to secede from the Union.

Lincoln's election ignited the Secession Crisis, in which seven states, fearing Lincoln would abolish slavery, seceded from the Union. These seven, in order, were:

Lincoln was inaugurated on 4 March 1861, and the Confederate forces under P. G. T. Beauregard fired on Fort Sumter on 12 April, starting the American Civil War. After this, four other states seceded:

Texas, Virginia, and Tennessee have two dates indicating the decision to secede and the date the vote passed. These eleven became the Confederate States of America with Jefferson Davis as their president.

Between December 1860 and April 1861, tensions continued to rise, finally breaking into armed conflict when Confederate forces fired on the Union Fort Sumter just off the coast of South Carolina. Lincoln then called for a mobilization of militia to put down the insurrection.

Winfield Scott, Commanding General of the US Army and hero of the Mexican-American War, developed the "Anaconda Plan" to put down the rebellion: the South would be surrounded by Union land and naval forces, cutting them off from outside commerce, and then 'squeezed' until they surrendered. Although initially criticized, Scott's plan would remain in effect throughout the war.

The first land engagement was the skirmish known as the Philippi Races (Battle of Philippi, 3 June), a Union victory, but the first major clash was the First Battle of Bull Run/First Manassas (21 July), won by the Confederates, which ended Union hopes of a quick resolution to the conflict. Scott was too old to command in the field, and so Lincoln appointed Major General Irvin McDowell to command, who was then replaced by General George B. McClellan.

The Border States – Delaware, Kentucky, Maryland, Missouri, and, after 1863, West Virginia – which did not support "Lincoln's War" but rejected secession – tried to remain neutral but could not avoid the conflict as it progressed throughout 1861. Pro-slave and pro-free factions in these states fought their own internal wars while, at the same time, being periodically drawn into the wider engagement.

Native American nations were also drawn into the war, which divided not only the larger bodies but also smaller bands. Native Americans, including the Cherokee, Choctaw, Creek, and Seminole, fought for the Confederacy, hoping to win back their ancestral lands, but many different nations joined the war in 1861 on both sides and fought to the end.

American men and women of all ages, and on both sides, as well as those from foreign countries, joined the war effort in 1861. Women not only served as nurses but also as spies, saboteurs, and some even went into battle disguised as men.

In the Eastern theater of the war, early battles were inconclusive. McClellan launched his Peninsula Campaign in March of 1862 with the goal of capturing Richmond, Virginia, the capital of the Confederacy, and ending the war quickly. He faced forces under General Joseph E. Johnston, whose clever tactics, combined with McClellan's hesitancy, thwarted the campaign.

Johnston was wounded at the Battle of Seven Pines in May and was replaced by Robert E. Lee, whose more direct tactic of offense won the Seven Days' Battles (25 June to 1 July), ending the campaign. Although many of these battles were actually inconclusive, Lee's tactics proved superior, repelling McClellan's advances until the latter gave up and withdrew. Lee's commanders, James Longstreet and Stonewall Jackson, played a significant role in stopping McClellan's advance.

The Eastern theater also saw the Battle of Hampton Roads in Virginia (8 to 9 March), also known as the Battle of the Monitor and the Merrimack, the first naval battle of ironclad warships. Although inconclusive, it marked a turning point in naval warfare.

In the Western theater, Ulysses S. Grant took Fort Henry (6 February) and then Fort Donelson (16 February), demanding unconditional surrender and gaining the nickname "Unconditional Surrender Grant." Grant's victories forced Confederate General Nathan Bedford Forrest to retreat, leaving central Tennessee open to Union control and cutting off essential supplies to the Confederacy.

Grant also won the Battle of Shiloh (6 to 7 April) in Hardin County, Tennessee in which Confederate General Albert Sidney Johnston was killed and replaced by P.G.T. Beauregard. Shiloh was the costliest victory of the Civil War until the Battle of Antietam/Battle of Sharpsburg in Maryland on 17 September, the single bloodiest day of the entire war. The Union strategic victory at Antietam (the battle was inconclusive, but the Confederates left the field) enabled Lincoln to issue the preliminary draft of the Emancipation Proclamation, freeing all slaves held within the Confederate States.

McClellan was replaced by General Ambrose Burnside, who proved to be as incompetent overall as he had been at Antietam, where his refusal to consider options in crossing what became known as "Burnside's Bridge" added significantly to Union casualties. After the costly Union defeat at the Battle of Fredericksburg in Virginia (11 to 15 December), Burnside was replaced by Major General Joseph Hooker.

On 1 January 1863, Lincoln formally issued the Emancipation Proclamation, which, although only freeing slaves in the states in rebellion, opened enlistment in the Union army to free Blacks of the North. The first Black regiment had been organized in September 1862, but now there was widespread enlistment, encouraged by high-profile figures including Harriet Tubman and Frederick Douglass. In February 1863, the famous 54th Massachusetts Regiment was formed, one of the earliest African American regiments to see action in the war and probably best known today from the movie Glory (1989), featuring their doomed assault on Fort Wagner in July 1863.

In March, Lincoln signed the Conscription Act, the first military draft in American history, adding many more soldiers to the Union effort. General Hooker proved himself an able commander until the Battle of Chancellorsville in Virginia (30 April to 6 May), during which General Lee divided his troops for a complete victory. Even so, this cost Lee one of his greatest generals when Stonewall Jackson was accidentally shot by his own men, mistaking his entourage for a Union advance. Jackson was severely wounded and later died of pneumonia on 10 May.

In the Western theater of the war, Ulysses S. Grant and William Tecumseh Sherman continued the Vicksburg Campaign that had begun on 29 December 1862. The goal was to capture the city of Vicksburg, which controlled the last Confederate-held section of the Mississippi River. Grant's masterful campaign finally took Vicksburg on 4 July, cutting off the Eastern theater of the Confederacy from the West.

After Chancellorsville, Hooker was replaced by Major General George Meade, who was in command when Lee launched his invasion of the North. The two armies met at the Battle of Gettysburg (1 to 3 July), the war's bloodiest conflict, costing over 50,000 lives in three days. On the third day, Confederate General George Pickett led an assault on the Union center, known as Pickett's Charge, which is considered the high-water mark of the Confederacy. The charge was broken, and Lee was defeated. Meade's victory at Gettysburg marked a turning point in the war, especially coupled with Grant's success at Vicksburg the next day.

Still, the war raged on as Union forces pressed an offensive to take the Confederate states by storm. The Battle of Chickamauga (18 to 20 September) was the conclusion of Union Major General William Rosecrans' attempt to break the Confederate Army of Tennessee under Confederate General Braxton Bragg. Chickamauga was a Confederate victory, despite the heroic defense mounted by Union Major General George Henry Thomas, and the costliest to both sides in the Western theater and second only to Gettysburg in the number of casualties in a single battle in the entire war.

Lincoln delivered his famous Gettysburg Address on 19 November, rededicating the nation to the cause of union, but the war continued on into another year

The year began with skirmishes until February, when Union General Sherman began his Meridian Campaign in Mississippi. On 17 February, the Confederate submarine H. L. Hunley sank the USS Housatonic. Although the Hunley also sank at the same time, taking its entire crew with it, the attack marked the first time a submarine sank a warship, changing the face of naval warfare.

On 9 March, Grant was commissioned Lieutenant General and General-in-Chief of the US Army, and in May began his Overland Campaign (also known as the Wilderness Campaign, May-June 1864). The campaign began with the Battle of the Wilderness (5 to 6 May), which, though inconclusive, cost Lee men he could not spare. The Battle of Spotsylvania Courthouse (8 to 21 May) followed, and while this was being fought, Union Major General Philip Sheridan drove his cavalry against that of J. E. B. Stuart, who was mortally wounded at the Battle of Yellow Tavern (11 May) and died the next day.

The Battle of Spotsylvania Courthouse was also inconclusive, but Grant refused to retreat, despite serious casualties. Every time he engaged with Lee, no matter the outcome, he regrouped and continued the campaign, even after a serious defeat at the Battle of Cold Harbor (31 May to 12 June).

In June, Union army engineers built a pontoon bridge across the James River, stretching 2,200 feet (670 m), the longest pontoon bridge of the war, enabling troops to support the Richmond-Petersburg Campaign (9 June 1864 to 25 March 1865), which included the Siege of Petersburg, a city vital to Confederate interests.

Upon Grant's promotion, General Sherman had taken his place in the Western theater and, coordinating with Grant, pursued a policy of total war, concluding that the conflict could only be won by destroying the Confederacy's ability to wage war. Lee was trapped at Petersburg, and Grant was engaged holding him there, and so, on 15 November, Sherman launched his Savanah Campaign, better known as Sherman's March to the Sea.

Following a "scorched earth" policy of destroying not only military facilities but also industries and civilian farms and homes, Sherman devastated Georgia between 15 November and 21 December and then began the Carolinas Campaign and did the same to South Carolina and, to a lesser degree, North Carolina. Earlier, in October, General Sheridan had conducted his successful campaign in the Shenandoah Valley, defeating Confederate Major General Jubal A. Early and, following Sherman's example, destroying any assets in the region that might enable the Confederacy in any way.

The Confederacy's great hope at this point was that Lincoln would lose the presidential election of 1864 and the incoming president would end the war. Lincoln, however, won the election on 8 November 1864, and the war continued on into the New Year, even though a Confederate victory at this point must have been impossible for anyone to imagine.

The early months of 1865 began as before, with more battles, but Southern hope was dissipating quickly. Sheridan defeated General Pickett at the Battle of Five Forks in Virginia on 1 April, seizing a vital supply line and forcing Lee to abandon Petersburg and Richmond to Union forces. Richmond was taken on 2 to 3 April by the Union XXV Corps, composed of African American troops.

Lee retreated to Appomattox Station, where he intended to resupply and resume hostilities, but Grant surrounded and defeated him at Appomattox Court House. Lee surrendered the Army of Northern Virginia to Grant on 9 April 1865, and, although hostilities continued, the war was over. The final engagement of the American Civil War was the Battle of Palmito Ranch in Texas on 13 May 1865, and the war's finish is now dated to 26 May 1865 when the Trans-Mississippi Department surrendered.

On 19 June 1865, Major General Gordon Granger arrived in Texas to deliver General Order No. 3, announcing emancipation to the slaves the Confederate States had sent there to keep them from being seized by Union forces. This event is now celebrated as Juneteenth.

Prior to the American Civil War, people in the United States identified themselves by their state. One was a Virginian, a New Yorker, a Texan, a Kansan; after the Civil War, people began to refer to themselves as "American" in a way they never had before.

Even though the war cost thousands of lives – many from disease rather than battle – and included in that death toll the assassination of President Abraham Lincoln – it ultimately drew the states together into exactly the kind of union Lincoln envisioned and encouraged people to fight to defend. The Civil War, far more than the American Revolution, gave birth to the United States of America.