Abraham Lincoln (February 12, 1809 – April 15, 1865) was the 16th president of the United States, serving from 1861 until his assassination in 1865. He led the United States through the American Civil War, defeating the Confederate States and playing a major role in the abolition of slavery.

Lincoln was born into poverty in Kentucky and raised on the frontier. He was self-educated and became a lawyer, Illinois state legislator, and U.S. representative. Angered by the Kansas–Nebraska Act of 1854, which opened the territories to slavery, he became a leader of the new Republican Party. He reached a national audience in the 1858 Senate campaign debates against Stephen A. Douglas. Lincoln won the 1860 presidential election, prompting a majority of slave states to begin to secede and form the Confederate States. A month after Lincoln assumed the presidency, Confederate forces attacked Fort Sumter, starting the Civil War.

As a moderate Republican, Lincoln had to navigate conflicting political opinions from contentious factions during the war effort. Lincoln closely supervised the strategy and tactics in the war effort, including the selection of generals, and implemented a naval blockade of Southern ports. He suspended the writ of habeas corpus in April 1861, an action that Chief Justice Roger Taney found unconstitutional in Ex parte Merryman, and he averted war with Britain by defusing the Trent Affair. On January 1, 1863, he issued the Emancipation Proclamation, which declared the slaves in the states "in rebellion" to be free. On November 19, 1863, he delivered the Gettysburg Address, which became one of the most famous speeches in American history. He promoted the Thirteenth Amendment to the U.S. Constitution, which, in 1865, abolished chattel slavery. Re-elected in 1864, he sought to heal the war-torn nation through Reconstruction.

On April 14, 1865, five days after the Confederate surrender at Appomattox, Lincoln was attending a play at Ford's Theatre in Washington, D.C., when he was fatally shot by Confederate sympathizer John Wilkes Booth. Lincoln is remembered as a martyr and a national hero for his wartime leadership and for his efforts to preserve the Union and abolish slavery. He is often ranked in both popular and scholarly polls as the greatest president in American history.