How was Reconstruction a failure?
Some historians would argue that, by some measures, Reconstruction was a success. The most famous historian of Reconstruction, Eric Foner, has called it an "unfinished revolution" in a book by that title. Where Reconstruction failed was in establishing a permanent foundation for black political, social, and especially economic equality in the South. On the last point, even most of Reconstruction's most radical boosters did not envision that black families would be the beneficiaries of land reform in the post-Civil War South. Most thought that freedmen would become "free labor," and work for white people for wages or in some other arrangement. What happened is that most African-Americans, for lack of other options, entered into sharecropping or tenant arrangements with white landowners. This trapped them in a cycle of debt and rural poverty that lasted more than a century in many regions.
Politically, serious efforts were made to establish and maintain voting rights for black men, but these efforts faltered in the face of organized white resistance. White "redeemer" governments began to retake control of Southern states as early as 1870, and the governments they established (with some notable exceptions) were for and by white men. Over time, enthusiasm for enforcing Reconstruction's mandates for equality faded. White organizations like the Ku Klux Klan made it possible for "Bourbon" Democrats to reestablish white supremacy in the South just a few years after the war that had fought to end slavery. "Race riots" and lynchings throughout the South terrorized black men, who found going to the polls to vote a "radical" ticket a dangerous proposition.
Many of the black institutions--churches, schools, associations, etc.--born during Reconstruction would survive, but they represented the only real outlets for African-American public life as Reconstruction came to an end. In less than two decades after Reconstruction, Southern states established Jim Crow laws, cementing in place a rigid form of segregation and white supremacy that made a mockery of the idealism of Reconstruction.