The leader of the march is Protestant M.P. Ivan Cooper (James Nesbitt), who is committed to nonviolent protest. The movie never stops to give the usual character-building exposition, yet full-fledged characters do emerge, defined by their grace–or disgrace–under pressure. We follow a 17-year-old Irish hothead (Declan Duddy) who has been arrested before for rioting; the cool, iron-fisted commander of the British forces, Major General Ford (Tim Pigott-Smith), a master of media manipulation; Brigadier MacLellan (Nicholas Farrell), the man who must carry out Ford’s belligerent orders, though they go against his conscience; the officers and soldiers out in the streets, itching for confrontation with the “hooligans” Ford wants to arrest. The movie builds with relentless force, but Greengrass doesn’t want slickness or sensationalism: there’s no music score, and the brief episodes fade to black and come full stop, giving the movie a lurching, awkward rhythm that’s meant to mirror the random, confusing reality of that sanguine afternoon. When the soldiers open fire on unarmed victims, the horror unfolds in terrifying fragments, leaving the viewer as disoriented as the crowd, which flees for cover from bullets that appear to come from nowhere. Perhaps the casting of pig-eyed Pigott-Smith is a bit lazy: he carries too much villainous baggage from previous roles (many will remember his evil turn in “The Jewel in the Crown”). But that’s a minor flaw in a film that brings history to life with an uncanny sense of realism. The massacre destroyed the Northern Irish civil-rights movement and played right into the hands of the IRA, violence begetting more violence. It’s sickeningly au courant.