After the Civil War, the US Constitution became a means to ensure that African-Americans had rights in American society. This was due to the passage of the following three amendments:
1) 13th amendment- This outlawed slavery or forced labor all over the US.
2) 14th amendment- This amendment states that any person born on US soil or naturalized has all of the rights/freedoms guaranteed in the US Constitution.
3) 15th amendment- This amendment states that a person cannot be stopped from voting based on race, color, or past servitude. Essentially, this makes it so that a government can't stop a person from voting based on the fact that they were a slave before this time period.