Abraham Lincoln (February 12, 1809 – April 15, 1865) was the 16th President of the United States. He successfully led the country through its greatest internal crisis, the American Civil War, preserving the Union and ending slavery. As the war was drawing to a close, Lincoln became the first American president to be assassinated. Before his election in 1860 as the first Republican president, Lincoln had been a country lawyer, an Illinois state legislator, a member of the United States House of Representatives, and twice an unsuccessful candidate for election to the U.S. Senate.

As an outspoken opponent of the expansion of slavery in the United States, Lincoln won the Republican Party nomination in 1860 and was elected president later that year. His tenure in office was occupied primarily with the defeat of the secessionist Confederate States of America in the American Civil War. He introduced measures that resulted in the abolition of slavery, issuing his Emancipation Proclamation in 1863 and promoting the passage of the Thirteenth Amendment to the Constitution, which passed Congress before Lincoln's death and was ratified by the states later in 1865.

Lincoln closely supervised the victorious war effort, especially the selection of top generals, including Ulysses S. Grant. Historians have concluded that he handled the factions of the Republican Party well, bringing leaders of each faction into his cabinet and forcing them to cooperate. Lincoln successfully defused the Trent affair, a war scare with Britain, in 1861. Under his leadership, the Union took control of the border slave states at the start of the war. Additionally, he managed his own reelection in the 1864 presidential election.

Copperheads and other opponents of the war criticized Lincoln for refusing to compromise on the slavery issue. Conversely, the Radical Republicans, an abolitionist faction of the Republican Party, criticized him for moving too slowly in abolishing slavery. Even with these road blocks, Lincoln successfully rallied public opinion through his rhetoric and speeches; his Gettysburg Address is but one example of this. At the close of the war, Lincoln held a moderate view of Reconstruction, seeking to speedily reunite the nation through a policy of generous reconciliation. His successor in the White House, Andrew Johnson, also wanted reconciliation among white Americans, but failed to protect the rights of newly freed slaves. Lincoln's assassination in 1865 was the first presidential assassination in American history. He has since consistently been ranked by scholars as one of the greatest U.S. Presidents.