Tavis Smiley's up to more than squabbling with Rev. Al Sharpton and hashing out the "Black agenda". Tonight, PBS will air the second episode of his four-part, year-long series of special reports. This one will try to unearth a part of Martin Luther King, Jr.'s radical legacy that's been largely scrubbed from our memory: His strong and controversial advocacy against the Vietnam war. Smiley zeros in on King's April 4, 1967, speech, in which he first publicly criticized the war, drawing scorn from a wide swath of news media and from the White House. Many had urged King to avoid the growing public debate about Vietnam. They feared -- rightly, it turns out -- that King would alienate important allies in the civil rights movement and distract from his central campaign against racial justice. King, however, believed no racial justice campaign could truly succeed without addressing what he called "the giant triplets of racism, materialism, and militarism."