Abraham Lincoln entered the Illinois legislature as a Whig in 1834 and developed a successful legal practice after admission to the bar. He served in Congress from 1847 to 1849 and criticized the Mexican War. The Kansas-Nebraska Act in 1854 revived his national political ambitions by reopening the question of slavery's expansion. Joining the Republican Party, he argued that slavery should be restricted while initially accepting its constitutional protection where it already existed.

The 1858 debates with Stephen Douglas increased Lincoln's visibility despite his failure to win a Senate seat. Elected president in 1860, he faced secession before taking office. The attack on Fort Sumter in April 1861 began the Civil War. He mobilized forces, managed a divided political coalition, and worked through repeated difficulties in finding effective commanders. Measures including suspension of habeas corpus raised enduring questions about wartime executive authority. His central initial goal was preserving the Union, but the war increasingly linked that goal to destroying slavery.

The Emancipation Proclamation took effect in January 1863, declaring freedom for enslaved people in designated rebellious areas. It did not immediately free every enslaved person in the United States; military action, self-emancipation, and later constitutional change were essential. Lincoln supported Black military recruitment and gradually reconsidered earlier views about colonization and citizenship. The Gettysburg Address connected national survival to equality and a renewed conception of democratic government. His administration also supported major legislation concerning banking, homesteads, land-grant colleges, and the transcontinental railroad.

Reelection in 1864 depended on military progress and sustaining the Union coalition. Lincoln pressed for the Thirteenth Amendment, approved by Congress in January 1865, to abolish slavery nationally. His second inaugural address acknowledged the war's relationship to slavery while calling for reconciliation, but Reconstruction remained unsettled. Union policy also involved severe harm to Indigenous peoples, and emancipation did not establish racial equality. After the principal Confederate surrender, John Wilkes Booth shot Lincoln at Ford's Theatre on April 14; he died the next morning. His career combined political argument, military leadership, and changing positions on slavery and citizenship.