Both Lincoln and Johnson had foreseen that the Congress would have the right to deny Southern legislators seats in the U.S. Senate or House of Representatives, under the clause of the Constitution that says "Each house shall be the judge of the...qualifications of its own members." This came to pass when, under the leadership of Thaddeus Stevens, those congressmen (called "Radical Republicans") who sought to punish the South refused to seat its elected senators and representatives. Then, within the next few months, the Congress proceeded to work out a plan for the reconstruction of the South quite different from the one Lincoln had started and Johnson had continued.  

Wide public support gradually developed for those members of Congress who believed that blacks should be given full citizenship. By July 1866, Congress had passed a civil rights bill and set up a new Freedmen's Bureau -- both designed to prevent racial discrimination by Southern legislatures. Following this, the Congress passed a 14th Amendment to the Constitution, which states that "All persons born or naturalized in the United States and subject to the jurisdiction thereof, are citizens of the United States and of the states in which they reside," thus repudiating the Dred Scott ruling which had denied slaves their right of citizenship.  

All the Southern state legislatures, with the exception of Tennessee, refused to ratify the amendment, some voting against it unanimously. In addition, in the aftermath of the war, Southern state legislatures passed black codes, which aimed to reimpose bondage on the freedmen. The codes differed from state to state, but some provisions were common. Blacks were required to enter into annual labor contracts, with penalties imposed in case of violation; dependent children were subject to compulsory apprenticeship and corporal punishments by masters; and vagrants could be sold into private service if they could not pay severe fines.  

In response, certain groups in the North advocated intervention to protect the rights of blacks in the South. In the Reconstruction Act of March 1867, Congress, ignoring the governments that had been established in the Southern states, divided the South into five districts and placed them under military rule. Escape from permanent military government was open to those states that established civil governments, took an oath of allegiance, ratified the 14th Amendment and adopted black suffrage.  

The amendment was ratified in 1868. The 15th Amendment, passed by Congress the following year and ratified in 1870 by state legislatures, provided that "The rights of citizens of the United States to vote shall not be denied or abridged by the United States or any state on account of race, color or previous condition of servitude."  

The Radical Republicans in Congress were infuriated by President Johnson's vetoes (even though they were overridden) of legislation protecting newly freed blacks and punishing former Confederate leaders by depriving them of the right to hold office. Congressional antipathy to Johnson was so great that for the first time in American history, impeachment proceedings were instituted to remove the president from office.  

Johnson's main offense was his opposition to punitive congressional policies and the violent language he used in criticizing them. The most serious legal charge his enemies could level against him was that despite the Tenure of Office Act (which required Senate approval for the removal of any officeholder the Senate had previously confirmed), he had removed from his Cabinet the secretary of war, a staunch supporter of the Congress. When the impeachment trial was held in the Senate, it was proved that Johnson was technically within his rights in removing the Cabinet member. Even more important, it was pointed out that a dangerous precedent would be set if the Congress were to remove a president because he disagreed with the majority of its members. The attempted impeachment failed by a narrow margin, and Johnson continued in office until his term expired.  

Under the Military Reconstruction Act, Congress, by June 1868, had readmitted Arkansas, North Carolina, South Carolina, Louisiana, Georgia, Alabama and Florida, to the Union. In many of these seven reconstructed states, the majority of the governors, representatives and senators were Northern men -- so-called "carpetbaggers" -- who had gone South after the war to make their political fortunes, often in alliance with newly freed African Americans. In the legislatures of Louisiana and South Carolina, African Americans actually gained a majority of the seats. The last three Southern states -- Mississippi, Texas and Virginia -- finally accepted congressional terms and were readmitted to the Union in 1870.  

Many Southern whites, their political and social dominance threatened, turned to illegal means to prevent blacks from gaining equality. Violence against blacks became more and more frequent. In 1870 increasing disorder led to the passage of an Enforcement Act severely punishing those who attempted to deprive the black freedmen of their civil rights.