Could you summarize the plot?
World War I. The lines between the English and the Germans have been static for months. But there has been news that the Germans are suddenly retreating, so two battalions are in pursuit.
Headquarters discovers from aerial reconnaissance that, however, those two battalions are being lured into a trap. It becomes necessary to send two men across No-Man's Land, through the several miles the Germans have left open, and catch up to the two battalions before they launch their attack into what will be a slaughter. They are to deliver orders from the general to call off the attack--scheduled for the next morning. One of the two men has a brother in one of the battalions, so he has a particular reason to go on the mission. This reminded me of the plot of a WWII story I read in high school, "Going to Run All Night" by Harry Sylvester.
Although the pair are not expected to meet much opposition (the Germans have "retreated," after all), there is still a major possibility of running into snipers or small patrols. The territory they have to maneuver though has been contested back and forth for months, so the land has been cratered by artillery from both sides and is littered with decomposing corpses from both sides.
So many men killed anonymously. Their bodies will never be identified. Their loved ones will never know how, and perhaps not even where they died. Those many anonymous corpses give a special poignancy to the end.
But there is also a clear Ecclesiastes moral to the story. Even if they manage to save those soldiers today, tomorrow there will be another battle, and another, and another. And we in the audience know what those soldiers don't: Not even this horror will be the "war to end all wars." In only twenty years, they will be fighting for the same countryside again.
The cinematography technique has gotten a lot of media attention. It is shot as though the entire movie is one long, continuous take, a steady, unblinking eye on the protagonists, sometimes leading them, sometimes following them, sometimes circling them, sometimes running, crawling, swimming beside them, but never blinking. The effect is to put the viewer right into the action with them, as though there is no fourth wall, and it emphasizes the urgency in every minute of the movie.
Of course, it's not really a single two-hour-long take, but it is composed of many very, very long takes. Some of them are as much as seven or eight minutes long of men in constant forward motion. They had to rehearse the movements months before they built the sets (which included a full mile of trenches), and then design the cinematography to fit the timing of the movement.
And because the actual takes were so long, the actors had to know their lines and movements cold. In some cases, errors were left in. One actor said, "If you made a mistake, you didn't stop unless the director called 'cut'."
But all that work was extremely effective. Sitting in the audience, I didn't dare glance down to grab a nacho.