OVER THE YEARS, an army of statues has been deployed in the parks, circles and squares of the nation’s capital, many of them commemorating men who played a role in what should have been the liberation of the African people in America. But the fervent hope that accompanied the end of slavery in this country was betrayed over subsequent decades. The newest monument honors the man who rekindled that hope with his insistence, a century after the Civil War, on freedom, equality and nonviolent change.

The Rev. Martin Luther King Jr.’s message was delivered most memorably on Aug. 28, 1963, on the steps of the Lincoln Memorial. The 1963 March on Washington was testimony to how Abraham Lincoln’s vision had been betrayed, how freedom for black...