The American Civil War (1861–1865) was an armed conflict between the United States federal government and eleven southern states that declared secession and formed the Confederate States of America. It remains the deadliest war in American history, with scholarly estimates of total deaths — Union and Confederate soldiers combined, plus civilian losses — ranging from approximately 620,000 to over 750,000, with more recent census-based analyses suggesting figures in the range of 698,000 to 750,000.¹⁾ The war resulted in the preservation of the federal union, the abolition of slavery, and the beginning of the Reconstruction era. Its causes, conduct, memory, and legacy remain subjects of active historical debate.

Sectional tensions between northern and southern states over slavery, tariffs, territorial expansion, and the balance of federal and state authority had accumulated for decades before the war's outbreak. The election of Abraham Lincoln in November 1860 on a platform opposing the expansion of slavery prompted South Carolina to secede in December 1860, followed by six additional Deep South states before Lincoln's inauguration. The Confederate attack on Fort Sumter, South Carolina, in April 1861 triggered Lincoln's call for federal troops and the secession of four more Upper South states: Virginia, Arkansas, Tennessee, and North Carolina. Four slave-holding border states — Missouri, Kentucky, Maryland, and Delaware — remained in the Union.

The war's principal theaters were the Eastern Theater (Virginia and the mid-Atlantic), the Western Theater (the Mississippi Valley and the Deep South), and the Trans-Mississippi Theater. Major engagements included the First and Second Battles of Bull Run, the Battle of Antietam, the Siege of Vicksburg, the Battle of Gettysburg, and Sherman's March to the Sea. The Confederacy's effective military resistance collapsed with Lee's surrender at Appomattox Court House on 9 April 1865, followed by the surrender of remaining Confederate land forces through June of that year, with isolated naval resistance continuing until November 1865.

The Thirteenth Amendment to the Constitution, abolishing slavery throughout the United States, was ratified in December 1865. For a fuller treatment of the conflict's origins, course, and aftermath, see American Civil War — History.

Professional historians regard the factual outline of the war — its dates, principal campaigns, casualty figures, and constitutional outcome — as well established. Several interpretive questions, however, remain contested among scholars and the general public alike.

The relative weight of slavery versus other factors (states' rights, economic divergence, territorial disputes) as causes of secession is a central point of debate; see American Civil War — Causes Debate. The military competence and strategic decisions of commanders on both sides, including Lee, Grant, Sherman, and Jefferson Davis, are subjects of ongoing reassessment. The role of African American soldiers — approximately 180,000 served in the Union Army — and of enslaved people's resistance and self-emancipation in shaping the war's outcome is an area of active scholarship.²⁾ The human costs of the conflict, including the experiences of civilians, prisoners of war, and the wounded, have received increased attention in recent decades.

The war's memory and commemoration — including the rise of the “Lost Cause” narrative, monument controversies, and debates over the Confederate battle flag — are matters of ongoing public and scholarly dispute; see American Civil War — Memory Debate.

Broad consensus exists among professional historians that slavery was the central cause of Southern secession and, consequently, of the war; the Confederate states' own secession documents cited the preservation of slavery explicitly. Dissenting interpretations, including those emphasizing states' rights or economic factors as independent primary causes, are addressed on the American Civil War — Causes Consensus page.

Slavery as Central Cause The view held by most academic historians: that the defense and expansion of slavery was the animating cause of Southern secession and the war.

States' Rights as Central Cause The view, more prevalent in popular than academic discourse, that the war was primarily a constitutional conflict over the sovereignty of states relative to the federal government, with slavery secondary or incidental.

Lost Cause Interpretation A historiographical tradition, originating in the postwar South, holding that the Confederate cause was noble, that Confederate armies were heroically outmatched, and that slavery was a benign or peripheral institution. Contested by most professional historians, who classify it as a post-hoc revisionist narrative, but influential in American memory and culture through much of the twentieth century.

Revisionist and Reconciliationist Views A range of postwar and twentieth-century interpretations that minimized the role of slavery and emphasized shared martial valor between North and South as a basis for national reunion.

Emancipationist View Emphasizes the war as fundamentally a struggle over human freedom, centering the agency of enslaved people and African American soldiers in shaping the conflict's outcome and meaning.

• American Civil War — History
• American Civil War — Causes Debate
• American Civil War — Memory Debate
• American Civil War — Causes Consensus
• Reconstruction
• Confederate States of America
• Emancipation Proclamation
• Thirteenth Amendment
• Viewpoint: Slavery as Central Cause
• Viewpoint: States' Rights as Central Cause
• Viewpoint: Lost Cause Interpretation
• Viewpoint: Revisionist and Reconciliationist Views
• Viewpoint: Emancipationist View
• Gettysburg Address

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