The Reconstruction Era (1865-1877) would be considered a failure because it fails to protect the rights of newly freed slaves. Even though the 13th (which gets rid of slavery), 14th amendment (states that people born in the US are automatically citizens), and 15th amendment (government can not stop a person from voting based on race) are passed in hopes of helping these citizens, southern states find ways to ensure that African-Americans do not have the rights promised to them by the federal government.
A perfect example of this would be the creation of the Ku Klux Klan. This organization of white supremacist use terror and physical violence to prevent African-Americans from voting. Law enforcement officials in the south often let this group cause physical harm to African-Americans without punishing them. Another example of this would be the creation of literacy tests and poll taxes. These obstacles made it more difficult for African-Americans to vote in elections. As you can see, the Reconstruction era truly failed to protect and help African-Americans.
