The American Civil War was the armed conflict fought in the United States from 1861 to 1865 between the Union and the Confederacy. The immediate political crisis concerned the secession of eleven Southern states after the election of Abraham Lincoln in 1860, but the fundamental issue underlying the sectional conflict was slavery and its relationship to federal power, territorial expansion and the political future of the United States.
Seven states seceded before Lincoln's inauguration, and four more joined after the attack on Fort Sumter in April 1861. The Confederate States of America established a separate government under Jefferson Davis, while the Union sought to preserve the United States. The war began as a conflict over secession and national authority, but emancipation became increasingly central to Union war aims.
The Emancipation Proclamation of 1863 changed the legal status of enslaved people in Confederate controlled territory and allowed large numbers of Black men to enlist in the Union forces. The Union victory in 1865 destroyed the Confederacy and led to the abolition of slavery through the Thirteenth Amendment.
The roots of the Civil War extended over decades. Slavery had existed since the colonial period and had become deeply integrated into the Southern agricultural economy, especially through cotton production. The expansion of the United States into western territories repeatedly raised the question of whether slavery would be permitted in new states and territories.
The Missouri Compromise of 1820, Compromise of 1850, Kansas Nebraska Act of 1854 and Dred Scott decision of 1857 all attempted or failed to resolve these disputes. The Republican Party emerged in the 1850s partly around opposition to the expansion of slavery into western territories. Abraham Lincoln opposed the extension of slavery but initially did not campaign in 1860 on immediate nationwide abolition. Southern secessionists, however, increasingly viewed Republican political power as a threat to slavery's future. Declarations of causes issued by seceding states explicitly identified slavery and its protection as central concerns. Thus the conflict involved constitutional questions concerning federal authority, but slavery was the principal institution around which those sectional disputes were organised.
The immediate crisis began after Lincoln's election. South Carolina seceded in December 1860, followed by Mississippi, Florida, Alabama, Georgia, Louisiana and Texas before Lincoln took office. The seceding states formed the Confederate States of America in February 1861 and selected Jefferson Davis as president. Lincoln rejected the legality of secession and maintained that the Union was perpetual.
Fort Sumter, located in Charleston Harbor, became the focal point of the crisis because Confederate forces demanded its surrender while the Lincoln administration attempted to resupply the garrison. Confederate forces bombarded the fort beginning on 12 April 1861, and the garrison surrendered two days later. Lincoln's call for volunteers prompted Virginia, North Carolina, Tennessee and Arkansas to secede. The conflict therefore expanded from a crisis involving several Deep South states into a war encompassing most of the slaveholding South. The border slave states of Delaware, Maryland, Kentucky and Missouri remained in the Union, although all experienced internal conflict.
The Union and Confederacy possessed different military and economic strengths. The Union had a larger population, greater industrial capacity, extensive railroad networks, substantial financial resources and control of the navy. The Confederacy possessed strong military leadership in several theatres, familiarity with local terrain and a defensive strategic position. Confederate leaders hoped that military success would secure independence and perhaps gain diplomatic recognition from Britain or France.
The Union initially pursued strategies centred on blockade, control of the Mississippi River and pressure against Confederate armies. General Winfield Scott's broad strategic concept eventually evolved into campaigns associated with the Anaconda Plan. The Confederacy attempted to exploit its cotton exports and diplomatic importance, but European powers generally avoided formal recognition. The naval blockade increasingly restricted Southern trade. Industrial and logistical differences became more significant as the war continued, although Confederate resistance remained formidable. The conflict required mass mobilisation, producing armies far larger than those maintained by the prewar United States.
Major campaigns included First Bull Run, Shiloh, Antietam, Gettysburg, Vicksburg, Chattanooga and the campaigns of Ulysses S. Grant and William Tecumseh Sherman in 1864 and 1865. The Battle of Antietam in September 1862 was the bloodiest single day in American military history, with enormous casualties on both sides. It provided Lincoln with the political circumstances to issue the preliminary Emancipation Proclamation.
The Union victories at Gettysburg and Vicksburg in July 1863 represented major strategic turning points, although the war continued for nearly two more years. Grant assumed command of all Union armies in 1864 and coordinated simultaneous offensives against Confederate forces. Sherman's Atlanta Campaign and March to the Sea targeted Confederate military infrastructure and economic resources. Grant's campaign against Robert E. Lee's Army of Northern Virginia eventually forced Lee's surrender at Appomattox Court House on 9 April 1865. Other Confederate armies surrendered subsequently.
Emancipation transformed the character of the war. The Emancipation Proclamation, effective from 1 January 1863, declared enslaved people in areas still in rebellion to be free, although it did not apply to loyal slave states or Confederate areas already under Union occupation. Its military significance included authorising Black enlistment in the Union armed forces, and nearly 200,000 African American soldiers and sailors eventually served the Union.
Enslaved people also undermined the Confederate labour system by escaping to Union lines, providing intelligence and seeking freedom. The proclamation also changed the diplomatic context by making abolition an explicit Union war objective. The Thirteenth Amendment, ratified in December 1865, abolished slavery throughout the United States except as punishment for crime after conviction. The transformation from a war primarily focused on preserving the Union into a war that also destroyed slavery was one of the conflict's most consequential developments.
The Civil War had immense human and economic costs. Approximately 750,000 military deaths is a widely cited modern estimate, although exact totals remain uncertain, and civilian deaths from disease, displacement, starvation and wartime violence added substantially to the toll. Disease killed more soldiers than combat, reflecting the limitations of nineteenth century medical knowledge and sanitation. Railways, telegraphs and industrial production became important military technologies, while ironclad ships and rifled weapons changed naval and battlefield conditions.
Total war practices became increasingly significant as armies targeted infrastructure, transportation networks and economic resources supporting the opposing war effort. The war also transformed federal power. The Union government introduced national taxation measures, a national banking system and extensive wartime economic regulation. The Homestead Act and Pacific Railway legislation also shaped western development during the conflict. The federal state emerged considerably stronger than it had been before 1861.
Reconstruction followed the military defeat of the Confederacy and became an extended struggle over citizenship, political power and the status of formerly enslaved people. Lincoln was assassinated by John Wilkes Booth in April 1865, only days after Lee's surrender, and Andrew Johnson became president. The Thirteenth, Fourteenth and Fifteenth Amendments abolished slavery, established constitutional citizenship and equal protection principles, and prohibited racial discrimination in voting on specified grounds.
However, Southern states developed Black Codes, and white supremacist violence sought to undermine Black political and economic autonomy. Congressional Reconstruction placed former Confederate states under military oversight and expanded federal protection of civil rights. The end of Reconstruction in 1877 coincided with the restoration of white Democratic political control across much of the South and the development of segregation and disenfranchisement. The Civil War therefore settled secession and abolished slavery but did not resolve racial inequality. Its constitutional and political consequences continued to shape American history long after 1865.
