Today, Martin Luther King, Jr.,’s work on civil rights is celebrated as bringing about one of the turning points of the twentieth century in America. But, in his own time, King was a divisive figure, unloved by millions of Americans—many members of government among them. The F.B.I. surveilled him constantly. President Lyndon Johnson worked with King to shape benchmark civil-rights legislation, but, after King spoke out against the Vietnam War, he was effectively alienated by the Administration. Meanwhile, J. Edgar Hoover’s agents at the F.B.I. began active measures to destroy King’s reputation and end his public influence, threatening to expose an extramarital affair. The documentary “MLK/FBI,” directed by Sam Pollard, examines this low point in the federal government’s abuse of power. Pollard tells Jelani Cobb that Hoover must have wondered, “ ‘How dare a Black man try to change the America I grew up in?’ The America he knew and loved was on a road to change. And he was totally against it.” Even today, as a leaked document shows, some within the F.B.I. see Black activists’ calls for justice and recognition as potential dangers to be watched carefully.