Barack Obama said in his final news conference that he planned to use his time off from politics “to do some writing.” I am hoping in his post-presidency, he begins to write a different racial history from the one he proclaimed from his presidential pulpit.

His is the story of America’s racial past that I am sure many Americans heard at celebrations of the Rev. Dr. Martin Luther King Jr. this month, as they stared down Donald J. Trump’s inauguration. While Mr. Obama granted in his farewell to the nation that “we’re not where we need to be,” he also said, “The long sweep of America has been defined by forward motion, a constant widening of our founding creed to embrace all, and not just some.”

When I watched the inauguration of President Trump on Friday, I did not see a nation in the forward motion of racial progress. I saw someone who pledged to take us away from progress and to new walls and more stop-and-frisk and law and order and the post-racial imaginary and to Jeff Sessionsand the alt-right.

Mr. Obama’s popular history of continuous racial progress does not explain how a candidate passionately endorsed by the Ku Klux Klan could succeed the first black president. It does not explain why millions of Americans felt the need to declare that black lives matter during that first black presidency. It does not explain why the mass incarceration of black and brown bodies followed the racial justice movements in the 1960s. It does not explain how Jim Crow could emerge out of the ashes of slavery, and why slavery expanded out West after Congress voted to outlaw the importing of slaves in 1807.

Mr. Obama has said, as he did in his farewell, that “for every two steps forward, it often feels like we take one step back.” Is that it? Is President Trump a step back? ...