The documentary “Nanking” begins with an unusual gambit: We see a group of actors, some of them recognizable, enter a room for what looks like a staged reading.

It is: These actors will read the words of real people who played a role in one of the 20th century’s cruelest, most barbaric episodes. Given the history of the last century, that’s saying a lot, but “Nanking” makes the case.

The film gives an account of the Japanese invasion of the Chinese city of Nanking (now called Nanjing) in 1937. In what became known as “the rape of Nanking,” Japanese soldiers slaughtered thousands of Chinese and systematically committed (according to official conclusions) 20,000 rapes against local women and children.

The death toll is officially estimated in the neighborhood of 200,000 people, although the Chinese have put it at more like 300,000. Both figures have been disputed by Japanese officials.

This episode, somewhat overshadowed by World War II, was revived in book form by the late Iris Chang’s “Rape of Nanking.” It is brought to sickening life by the film. Along with the actors’ spoken words, directors Bill Guttentag and Dan Sturman have collected a great deal of newsreel footage, including gruesome images of bodies and hospital wards.

All of this is calmly presented, which adds to the sense of horror. The actors do their work with gravity, but not melodrama. They include Jurgen Prochnow, reading the words of German businessman John Rabe; Mariel Hemingway, as Minnie Vautrin, American head of a missionary school; and Woody Harrelson as Bob Wilson, an American surgeon.

The film spends perhaps too much of its time with the story of these expats in Nanking, who banded together in a desperate attempt to shelter thousands of fleeing locals. It is undeniably a fascinating story, full of ironies (Rabe was a Nazi, and calculated that the Japanese would not want to attack German nationals or embarrass their ally).

The emphasis on the story of these heroic Westerners is likely there to provide a sense of “Schindler’s List”-style inspiration: the flicker of moral light within an atrocity.

That’s understandable, although there’s little light that comes out of the story of the rape of Nanking. What dominates the film is the first-hand testimony from Chinese victims, witnesses to murder, torture and rape. Even more disturbing than the body count is the general sense of barbarism, of the Japanese Army treating the Chinese as animals.

It’s hard to watch “Nanking,” but it’s necessary. This film gives context to an understanding of World War II and the ease with which humanity can vanish in a war zone.