On Oct. 20, 1981, a band of militant zealots armed with automatic weapons tried to rob a Brink’s truck in a shopping mall in Nanuet in Rockland County, N.Y. Before it was over, two armored-car guards were shot and two police officers — one black and one white — were gunned down at a roadblock. The crime was one of the last spasms of ’60s-style, left-wing violence. To the militants, it was an “expropriation” for something they called the Republic of New Afrika, a place that existed mainly in their fevered dreams.