After Lincoln's election, the southern states responded by seceding from the Union, after which they had to be forcibly returned into the United States via the Civil War. However, many in the North viewed the early stage of Reconstruction as being lenient, while the South was perceived as acting in bad faith. Despite the passage of the Thirteenth Amendment, ending slavery, and the Fourteenth Amendment, guaranteeing legal protection, the Southern states passed various Black codes, placing numerous constraints on the newly won freedoms of the former slaves, keeping them under suppression.

In 1867, Congress, now under the control of the Radical Republicans, responded by passing the First Reconstruction Act. This has the effect of placing all the former Confederate states (with the exception of Tennessee, which had ratified the Fourteenth Amendment) under martial law. As part of the First Reconstruction Act, it was required that the former states write new state constitutions. Meanwhile, the former states were reorganized into five military districts, each under the control of a military general, while the court systems were replaced by military tribunals. In addition, approximately twenty thousand US troops were marched into the Southern states. Ratification of the Fourteenth Amendment was a requirement for their readmittance into the Union.

The First Reconstruction Act marks a turning point in the history of Reconstruction, marking the transition from the first, more lenient stage of Reconstruction (under the leadership of the Executive Branch) into the second stage, known as Radical Reconstruction. Among the accomplishments of this second stage were the successful ratification of the Fourteenth Amendment, granting citizenship to African Americans, as well as the ratification of the Fifteenth Amendment, extending voting rights to the former slaves.