Director David Zeiger’s superb documentary about the Vietnam War era’s GI protest movement is jammed with incident and anecdote and moves with nearly as much breathless momentum as the movement itself, which was spurred along by mimeographed fliers and newspapers written by soldiers and secretly distributed on bases and battlefields. Such rebellion within the ranks stunned military brass and got more than one GI court-martialed and sentenced to years in prison. Several of those men appear here, in interviews juxtaposed against remarkable archival footage of their respective protests and arrests. Elsewhere, Zeiger’s film — which probably doesn’t have a Fox News Channel airing in its future — offers glimpses of the anti-war stage shows organized (by Jane Fonda and Donald Sutherland, among others) as counterpoint to Bob Hope’s boost-the-troops extravaganzas, plus wrenching testimony from the 1971 “Winter Soldier” hearings in Detroit, at which more than 100 combat vets spoke publicly about what they’d seen and done in Vietnam. Although it reverberates with resonance to our current misadventure, Sir! No Sir! should stand the test of time, not just for its captivating history lesson, but for its dispelling of many a myth, from the one about returning vets being spat on at the San Francisco airport, to the one that says an individual acting from conscience can’t manage to change the world.