The Reconstruction era was a period in American history following the American Civil War (1861–1865); it lasted from 1865 to 1877 and marked a significant chapter in the history of civil rights in the United States. Reconstruction, as directed by Congress, abolished slavery and ended the remnants of Confederate secession in the Southern states. It proclaimed the newly freed slaves (freedmen; black people) citizens with (ostensibly) the same civil rights as those of whites; these rights were nominally guaranteed by three new constitutional amendments: the 13th, 14th, and 15th, collectively known as the Reconstruction Amendments. Reconstruction also refers to the general attempt by Congress to transform the 11 former Confederate states, and refers to the role of the Union states in that transformation.

Following the assassination of President Abraham Lincoln—who led the Republican Party in opposing slavery and fighting the war—Vice President Andrew Johnson assumed the presidency. He had been a prominent Unionist in the South but soon favored the ex-Confederates and became the leading opponent of freedmen and their Radical Republicans allies. His intention was to give the returning Southern states relatively free rein in deciding the rights (and fates) of former slaves. While Lincoln's last speeches showed a grand vision for Reconstruction—including full suffrage for freedmen—Johnson and the Democrats adamantly opposed any such goals.

Johnson's Reconstruction policies generally prevailed until the Congressional elections of 1866, following a year of violent attacks against blacks in the South. These included the Memphis riots in May and New Orleans massacre in July. The 1866 elections gave Republicans a majority in Congress, power they used to press forward and adopt the 14th Amendment. Congress federalized the protection of equal rights and dissolved the legislatures of rebel states, requiring new state constitutions to be adopted throughout the South which guaranteed the civil rights of freedmen. Radical Republicans in the House of Representatives, frustrated by Johnson's opposition to Congressional Reconstruction, filed impeachment charges; the action failed by just one vote in the Senate. The new national Reconstruction laws incensed many whites in the South, giving rise to the Ku Klux Klan. The Klan intimidated, terrorized, and murdered Republicans and outspoken freedmen throughout the former Confederacy, including Arkansas Congressman James M. Hinds.

In nearly all ex-Confederate states, Republican coalitions came to power and directly set out to transform Southern society. The Freedmen's Bureau and the U.S. Army both aimed to implement a free-labor economy to replace the slave-labor economy that had existed until the end of the Civil War. The Bureau protected the legal rights of freedmen, negotiated labor contracts, and helped establish networks of schools and churches. Thousands of Northerners came to the South as missionaries and teachers as well as businessmen and politicians to serve in the social and economic programs of Reconstruction. "Carpetbagger" became a derisive term used to attack supporters of Reconstruction who travelled from the North to the South.

Elected in 1868, Republican President Ulysses S. Grant supported congressional Reconstruction and enforced the protection of African Americans in the South via the Enforcement Acts recently passed by Congress. Grant used the Acts to combat the Ku Klux Klan, the first iteration of which was essentially wiped out by 1872. Grant's policies and appointments were designed to promote federal integration, equal rights, black immigration, and the Civil Rights Act of 1875. Nevertheless, Grant failed to resolve the escalating tensions inside the Republican Party between Northern and Southern Republicans (the latter group would be labeled "scalawags" by those opposing Reconstruction). Meanwhile, white "Redeemers", Southern Bourbon Democrats, strongly opposed Reconstruction.Eventually, support for continuing Reconstruction policies declined in the North. A new Republican faction emerged that wanted Reconstruction ended and the Army withdrawn—the Liberal Republicans. After a major economic recession in 1873, the Democrats rebounded and regained control of the House of Representatives in 1874. They called for an immediate end to the occupation. In 1877, as part of a congressional bargain to elect a Republican as president following the disputed 1876 presidential election, federal troops were withdrawn from the three states (South Carolina, Louisiana, and Florida) where they remained. This marked the end of Reconstruction.

Reconstruction has been noted by historians for many "shortcomings and failures" including failure to protect many freed blacks from Ku Klux Klan violence prior to 1871, starvation, disease and death, and brutal treatment of former slaves by Union soldiers, while offering reparations to former slaveowners but denying them to former slaves. However, Reconstruction had four primary successes including the restoration of the Federal Union, limited reprisals against the South directly after the war, property ownership for black people, and the establishment of national citizenship and a framework for eventual legal equality.