Jackson was given 100 hours of footage of varying levels of quality . “It was sometimes a duplicate of a duplicate of a duplicate,” he said.

Much of this material, of soldiers in training and then in the trenches, was shot for propaganda newsreels that would play in theaters between other movies. “It’s interesting to think that this footage could have been put between a cartoon and a Charlie Chaplin film, and accompanied by organ music,” said Jean Cannon, the co-curator of a 2014 World War I exhibition at the University of Texas at Austin. “So in some ways the war gets on the scale of entertainment.”

In fact, t he first feature-length documentary to depict combat, “The Battle of the Somme,” was released mid-war, in 1916, and drew nearly 20 million moviegoers.

For Jackson’s documentary, rather than sift through the archival footage to decide which scenes to use, he opted to restore all 100 hours first (working on that daunting three-year task with a New Zealand company, Park Road Post Production). Decades of scratches, dust and splotches were cleaned up, and the now-pristine material was donated back to the war museum.

There were other technological adjustments as well. Jackson’s goal was to reconnect audiences with the soldiers in a way even more intimate than “The Battle of the Somme” did. The footage had a herky-jerky feel because it had been shot on hand-cranked cameras that produced images at a much slower frame rate than modern audiences are used to. Jackson’s team retimed the footage, speeding up the frame rate, adding extra frames digitally and smoothing out the movement.