After the Civil War, there was a period called Reconstruction. The main goal of Reconstruction was to try to get the Confederacy back into the United States. Along with this, it meant the South had to give the same rights to former slaves that the North did, which meant Union troops were stationed in the South to make sure they were following these rules.
After Reconstruction ended though, many of the troops were sent back and could no longer watch over the South. This was because of the Compromise of 1877, which was the proper end to Reconstruction. Since there was nobody now to make sure African Americans were being treated like they were supposed to, they were often being treated terribly.
The Compromise of 1877 meant that African Americans could now be treated badly by Southerners, who already resented them and resented the Union for forcing them to give them rights. This caused the Jim Crow era, along with violence against African Americans.
