Civil War, director Alex Garland’s imagining of a divided states of America, appears to have touched a nerve, topping the weekend box office. For some cinemagoers, it may offer escapism. But for those who remember a spectacular act of domestic terrorism that struck the heartland a generation ago, it may feel more like destiny. “Our documentary will help people understand, how could we even get to a place where a commercial movie like Civil War can be released and it’s not sci-fi?” says Marc Levin, director of An American Bombing: The Road to April 19th, an HBO documentary about the...