What is the theme of Langston Hughes's poem "Let America Be America Again"?

This poem is a powerful message of how Hughes feels that America-which is supposed to be a land of dreams, equality and opportunity for all, no matter what race, religion or creed-has become a place where in fact that idea does not exist anymore.  Instead, it is a place where racism, greed, materialism and discrimination rule instead of opportunity and love.  He writes the poem not only from the perspective of a black man discriminated against (which he is), but from every form of down-trodden and abused person in America:  the "red man", "immigrant", "farmer", "worker", "poorest", all who are "bartered", "driven" and "pushed" from their dreams and rights.  He goes on to say that it is precisely this type of person who originally "dreamt our basic dream" of what America could and should be; it is precisely that type of person that was the "pioneer" of America.  He ends optimistically with a rallying cry, a call to action, heralding all these people to "redeem the land...and make America again."

Overall, the theme is one of unification against injustice.  Another possible theme is inequality that exists in America.  Another is corruption, and how it has destroyed the American dream.  It's a great poem-layered, profound, frustrating and inspiring all at once.  I hope that helps a bit!  Good luck!