On June 28, 1914, a young Serbian nationalist assassinated the presumptive heir to the Austro-Hungarian throne, thus starting World War I. That, at any rate, is the familiar way that the origins for this war have been shaped into a story, even if historians agree the genesis of the conflict is far more complicated. None of those complications and next to no history, though, have made it into “1917,” a carefully organized and sanitized war picture from Sam Mendes that turns one of the most catastrophic episodes in modern times into an exercise in preening showmanship.

The story is simple. It opens on April 6, 1917, with Lance Corporal Blake (Dean-Charles Chapman) and Lance Corporal Schofield (George MacKay), British soldiers stationed in France, receiving new orders. They are to deliver a message to troops at the front line who are readying an assault on the Germans, who have retreated. (Coincidentally or not, April 6 is the date that the United States formally entered the war.) The British command, however, believes that the German withdrawal is a trap, an operational Trojan horse. The two messengers need to carry the dispatch ordering the waiting British troops to stand down, thereby saving countless lives.

It’s the usual action-movie setup — a mission, extraordinary odds, ready-made heroes — but with trenches, barbed wire and a largely faceless threat. Blake jumps on the assignment because his brother is among the troops preparing the assault. Schofield takes orders more reluctantly, having already survived the Battle of the Somme, with its million-plus casualties. The modest difference in attitude between the messengers will vanish, presumably because any real criticism — including any skepticism about this or any war — might impede the movie’s embrace of heroic individualism for the greater good, which here largely translates as vague national struggle and sacrifice.