Had the Union soundly and quickly defeated the Confederacy, it's very likely that slavery would have remained. Instead the war dragged on, and the Union was forced to employ blacks in its ranks. The end result—total emancipation—was more a matter of military necessity than moral progress...
The United States did not save black people; black people saved the United States of America. With that task complete, our "ally" proceeded to repay its debt to its black citizens by pretending they did not exist.
Ta-Nehisi Coates has written one of the most powerful essays on American history, and the black experience within it, that I have ever read. I hesitated about including the words "and the black experience within it" because, at its core, it runs against the spirit of the truths that Coates writes.
American history is the black experience; without slavery there is no America; without slavery there is no democracy; without slavery there is no American dream. And yet this is a truth most Americans not only refuse to acknowledge, but actively run from. In that they are supported by the education system (wonderfully dissected several years ago in James Loewen's excellent book, Lies My Teacher Told Me).
Coats is responding to a series of essays between himself and Jonathan Chait, a writer with a brilliant political mind but who sometimes goes off on frolics of fancy with regard to America's place in the world. Coates expertly nails Chait's failure to understand the truth of American history to his failure to understand African-American history.
If you cannot grapple with that which literally built your capitol, then you are not truly grappling with your country. And if you are not truly grappling with your country, then your beliefs in its role in the greater world (exporter of democracy, for example) are built on sand. Confronting the black experience means confronting the limits of America, and perhaps humanity itself.
If you don't have time to read it now, bookmark and come back to it. You'll probably want to read it more than once in any case.