Photo: Jordan Gale/The New York Times/Redux It is hard to say when, exactly, the militarization of American life began. Troops have marched through towns and cities for centuries, called upon, at various points, to menace civilians or violently suppress protests. Fifty-five years ago, National Guardsmen shot and killed four unarmed college students at Kent State University; the troops had arrived to choke off protests against the Vietnam War. One dark question hanging over the nation today, as