1791, "action or process of reconstructing," from re- + construction. In U.S. political history sense (usually with a capital R-), from 1865. It had been used during the American Civil War in reference to reconstitution of the union.
The period after the Civil War in which the states formerly part of the Confederacy were brought back into the United States. During Reconstruction, the South was divided into military districts for the supervision of elections to set up new state governments. These governments often included carpetbaggers, as former officials of the Confederacy were not allowed to serve in them. The new state governments approved three amendments to the Constitution: the Thirteenth Amendment, which outlawed slavery; the Fourteenth Amendment, which had a provision keeping some former supporters of the Confederacy out of public office until Congress allowed them to serve; and the Fifteenth Amendment, which guaranteed voting rights for black men. Once a state approved the Thirteenth and Fourteenth Amendments, it was to be readmitted to the United States and again represented in Congress. The official end of Reconstruction came in 1877, when the last troops were withdrawn from the South.
Note: The program established for Reconstruction, largely the work of Republicans in the North, was far more severe than what President Abraham Lincoln had proposed before his assassination. Large numbers of white southerners resented being kept out of the “healing” of the nation that Lincoln had called for and were unwilling to give up their former authority. Ill feeling by former Confederates during Reconstruction led to the formation of the Ku Klux Klan and a long-standing hatred among southerners for the Republican party.