Final answer:
The violent period following the Civil War focused on the Southern states rejoining the Union is known as Reconstruction (1865-1877), a time of significant amendment to the U.S. Constitution and resistance against changes to the social order.
Step-by-step explanation:
The violent period after the Civil War in which the South was being readmitted into the Union is known as Reconstruction. This era spanned from 1865 to 1877 and was marked by a significant and often violent transition as the southern states were reintegrated into the United States. During Reconstruction, the Constitution was amended to abolish slavery (Thirteenth Amendment), define citizenship and equal protection under the laws (Fourteenth Amendment), and protect voting rights (Fifteenth Amendment). However, despite these efforts, there was extensive resistance from various groups in the South such as the Ku Klux Klan, which sought to maintain pre-Civil War social and racial hierarchies through violence and intimidation.
The period was also characterized by political strife, with Radical Republicans seeking a harsher approach to Reconstruction and conflict with President Andrew Johnson, who favored a more lenient reintegration of the Confederate states. Ultimately, while the Reconstruction era saw some progress, it faltered in creating long-term racial equality in America, and many of the gains made during this time were rolled back after the withdrawal of federal troops in 1877.