Critical to the promulgation and passage of these amendments in the Congress were the efforts of the northern Radical Republicans. They fought to make sure that slavery and discrimination in the Confederate states were not perpetuated in practice despite being officially outlawed, and that public officials in those states were loyal to the Union. However, both Presidents Lincoln and Johnson favored relatively lenient requirements for the readmission of Confederate states to the Union.
The result of these different views of readmission was considerable conflict between the White House and Congress over the requirements that should be imposed on the rebel states before they could be readmitted to the Union. Conflict over readmission focused on four core issues: formal nullification of the Acts of Secession, abolition of slavery, oaths of allegiance to the Union, and repudiation of debts incurred by the Confederacy and its member states.
When it appeared that the governments of the former Confederate states were seeking to reestablish the old order by introducing new restrictions on the former slaves, additional readmission requirements were added. These requirements sought to ensure equality of treatment and due process of the law for all citizens, and to institute black suffrage.
Lincoln's approach, the "10 Percent Plan," promised readmission when 10 percent of the number of a state's citizens eligible to vote in 1860 swore an oath of allegiance to the Union, and the state had abolished slavery. Johnson's approach included additional requirements, including ratification of the 13th Amendment (1865) abolishing slavery, but was still considered overly lenient by many northern Republicans in Congress.
In response to President Johnson's leniency, a number of former Confederate states began passing "Black Codes," laws that restricted the rights and freedoms of the former slaves. Republicans in Congress responded by passing the 14th and 15th Amendments, and implementing other strict policies to counteract efforts to restrict the political rights of newly-freed black citizens.