Langston Hughes wrote this poem in 1935 (it was published in 1936), during the middle of the Great Depression. As suddenly poorer white people pulled their money out of Harlem after the 1929 stock market crash, Harlem gradually withered, and the hopes that animated the Harlem Renaissance (of which Hughes was a major figure) began to wither too.
Hughes' poem reflects in its theme a sense that the idea behind America is being lost and is in need of revival. As he notes in the poem, the ideal of America as a "strong land of love" without kings or tyrants, a land of "Liberty" and "Equality," has never been realized for black people like him, nor for immigrants, who live in the land where the strong "crush the weak," nor for the "red man," the "farmer," or the "worker." He cries out that he and millions like him are not "free."
Despite the fact that the dream of what America is supposed to represent has never been enacted for most Americans, Hughes nevertheless calls for it to be revived and to be implemented in the real lives of average American people. It is a good dream, one worthy of pursuing and of bringing to fruition for the first time. He writes,
O, let America be America again—
The land that never has been yet—
And yet must be—the land where every man is free.
The theme of the poem is one of hope that the future can be better than the past if our country starts to live up to its ideals.
When the speaker instructs the reader to "Let America be America again," he implies that America now is not the same as America when it was first founded, when there were dreams of equality and justice and hope for everyone. He wants America to be like that America again. However, he seems to lower his voice, at first, in parentheses and declares that this America never actually existed to him.
The subtext, here, is that America is not a place where equality and justice and hope are offered to everyone. The speaker asks that America once again become the land of dreamers and love and freedom, where none will be oppressed by any other group. However, for the speaker, this America has never actually existed. He has never felt equal or free and has always felt oppressed. He implies that even when America was at its best, people like him still never had the opportunity that other, privileged people enjoyed. We learn that the speaker is a symbol of the poor white person, the black person, the Native American, the immigrant, and all those who come with hope and find that it is the same in America as everywhere else: the "mighty crush the weak." He is the exploited, the hungry, and one who has nearly lost his hope, despite the fact that it is people like him who built America.
The previous response already touched on many of the important themes in this poem, so I'll just add a couple more. This poem is especially chilling in the face of our current political climate. Other themes of this poem include the silencing of marginalized voices, the idea that America is a country founded on deception, and the idealization of America vs. the reality of America.
Regarding the silencing of marginalized voices, such as the voices of people of color and poor people, Hughes brings out this theme with his use of parentheses and italics, especially early on in the poem. Hughes includes stanzas that assert that we need to let America "be the dream it used to be," and that America should return to being "that great strong land of love." However, between these stanzas, Hughes weaves a second voice that says, "(America never was America to me)" and "Say, who are you that mumbles in the dark? And who are you that draws your veil across the stars?" This second voice is almost a whisper or an aside--a mumble in the dark. Hughes uses parenthesis and italics to make it seem like this second voice is almost being drowned out by the other, "louder" voices that want America to be great again. The juxtaposition of these two voices depicts the ways in which the marginalized voices are silenced and ignored in America.
The juxtaposition of these opposing voices also speaks to the second and third themes: the idea that America is a country founded on deception, and the idealization of America vs. the reality of America. The opposing voices, which almost seem to be arguing with each other in the poem, imply that perhaps America never was "great," that it never actually was the dreamland that we think it is, and that it has always been a country founded on deception, violence, racism, and classism. Hughes also tells us this more directly in the lines "O, let America be America again— / The land that never has been yet—"
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!