This was the reconstruction of the States into one nation after the Civil War. This raised a few questions: on what conditions would the Confederacy be allowed back into the Union? Who would establish these terms? What happened to freed blacks in the South? What exactly did the abolition of slavery mean for status between black and white men? After the Civil War, President Abraham Lincoln wanted to transition the seceded southern states back into the Union.
Lincoln appointed provisional military governors to the succeeded states which attempted to subdue/replace southern Confederate governments that supported secession--many states however did not receive enough votes. Lincoln also established the Freedman's Bureau, which assisted freedmen with legal advocacy, schooling, family relocation, employment, etc. He soon after was assassinated leading to Johnson being president.
Johnson pardoned most of the fighters and the Radicals won majority in Congress, while allowing southern states to reconstruct their own constitutions with the exception of abolishing slavery/denouncing secession; yet many southern states had already enacted Black Codes, declaring black people to be vagrant if unemployed/without permanent residence--therefore they could be arrested, fined, etc.
Civil Rights Bill (that guaranteed citizenship to all born in the US) and the 14th Amendment were passed shortly after. Many southerners disagreed with these decisions and white supremacist organizations were born/violently terrorized the Freedmen's Bureau, which eventually was forced to close. In many ways the Reconstruction period did not fulfill its original goal of readmitting the Southern states back into the union/stabilizing the equality of rights of African Americans. Once troops finally left the South, the southern states had realigned with their old values/retook control over their states