Historians find it hard to be peddlers of hope. A classic New Yorker cartoon shows a bearded historian lament to his psychiatrist: “Those who fail to study history are doomed to repeat it, but those who do study history are doomed to stand by helplessly while everyone else repeats it.” With much wisdom comes much sorrow, saith the psalmist. History is the story of human error battling plaguesome if not overwhelming structural forces, and things generally not going as well as we wish they might have.

And then there are the massacres. When I march in a demonstration and begin to chant, “The people, united, can never be defeated,” it makes me want to lie down in a puddle of tears. Historians might chant, “The people—united, deluded, indifferent, bamboozled—have often been defeated.” Of course, then we’d all give up, and how would that work out? My political life as a professional historian from 1994 to the present offers at least one notable lesson: If you think that things can’t get worse, you’re sorely mistaken.

To tell the sunnier story is a slide toward futility and perhaps a kind of insanity, a march into a circus mirror. This also leads toward disillusionment for those who would model themselves on Martin Luther King, Jr., and find they cannot live up to his legend. (Neither did he, for that matter.) Forcing the happy story means embracing delusions about who Americans have been, which inevitably lead to delusions about who Americans are. You may as well lie to your psychiatrist. “An invented past can never be used,” James Baldwin writes, “it cracks and crumbles under the pressures of life like clay in a season of drought.”

Traditions of struggle; now, those are a different matter. They are real and they matter and they must be embraced; they speak to our highest aspirations as a species. “The struggle of humanity against power,” writes Milan Kundera, “is the struggle of memory against forgetting.” It would be both misguided and mistaken to leave those traditions out of the story. They are a force in history and often a source of hope, in personal lives and sometimes even in works of history. But again and again, things fall apart, the best hopes are dashed, and history does not offer a happier lesson very often.

Sure, as the poet Seamus Heaney said, sometimes “hope and history rhyme,” and all that. But the poet also said in the previous lines, “History says don’t hope on this side of the grave / But then, once in a lifetime / The longed for tidal wave / Of justice can rise up,” and that right there tells you that the poet, though he hears history’s lecture, is nevertheless smoking moonbeams. Anytime a writer uses the metaphors of meteorology to describe human history, he or she simply has no idea what happened. ...