IT WAS 50 years ago this August that the Rev. Martin Luther King Jr. closed his speech on the steps of the Lincoln Memorial with his rendering of a dream he had for the country’s future. The soaring final sentences were somewhat extemporaneous — he let his emotions and sense of the occasion carry him past parts of the prepared text and on to the right words, concluding with the rousing “Free at last, free at last, thank God almighty we are free at last.” It was an exultant moment for much of this country, and in the national memory it has acquired the gauzy image of a happy ending to our long struggle with racial inequality and bigotry. Less vibrant in memory is an image from less than three weeks later: four girls dressed all in white because they were to lead youth day services at their Birmingham, Ala., church, their lives suddenly ended by a racial terrorist bombing.

“During the short career of Martin Luther King Jr., between 1954 and 1968, the nonviolent civil rights movement lifted the patriotic spirit of the United States toward our defining national purpose,” writes Taylor Branch, a chronicler of those years. But it was a hard lifting. In the years after the dream speech there were racially motivated murders in the South and riots in large cities in the North. Dr. King, who had emerged as a national figure amid the moral clarity of the Montgomery, Ala., bus boycott, found himself under attack from others in the civil rights movement for not pushing hard or fast enough, from the emerging black-power forces for being insufficiently “militant” and from people who disapproved of his emerging stands on the Vietnam war or economic issues. And then the King years ended in yet another atrocious act of violence — his assassination at a Memphis hotel. “Most of us will be grandparents before we can lead normal lives,” said one leader at the height of the civil rights movement in the early 1960s. He meant that the striving and agitating and the demonstrations were going to have to go on for a long, long time.

Today, on the national day set aside to honor Dr. King, an African American president will ceremonially begin his second term. Like presidents before him, he has had his ups and downs, mistakes and triumphs. There is, to be sure, an element of bigotry among some of his enemies, but in general it has had a kind of cowardly, subterranean quality to it. President Obama was assailed mostly for what his critics thought were wrong policies or judgments. In the end, as always, the final verdict was given at the polls; the president was reelected, and his inauguration will be celebrated today — not quite with the rapturous enthusiasm of four years ago but rather with something resembling blessed normality.

Dr. King’s words “Free at last, free at last” were specifically addressed not just to black Americans but to people of all faiths, colors and persuasions. He knew that they were all in need of liberation from the cruel customs and habits of the nation’s past, which held back every one of us in one way or another. He had a sense of the moment and, perhaps more important, an understanding of that past and of the “national purpose” that shone through its darkest decades and that could, he believed, some day be attained.