It Takes Two to Tangle: The Violin?s father-son duo is drawn into dirty politics.

There’s a chance you might have learned a few facts about Japan’s invasion of Nanking, China, while studying World War II in school or zoned out in front of the History Channel. In 1937, you might recall, the Japanese bombed the capital of China at the time until it fell, then tortured, killed, and raped some 200,000 citizens within six weeks. Another 200,000 found refuge in the “Safety Zone,” a small piece of land established and guarded by ­Westerners—among them a Nazi businessman—who sought to spare the Chinese too poor to flee as the Japanese soldiers adopted what was called the Three Alls Policy: Kill all, burn all, loot all. To this day, some Japanese dispute these numbers and even deny the atrocities themselves; it’s the Asian Holocaust.

You may think you know these things, but until you’ve seen Nanking, you have no idea.

In Bill Guttentag and Dan Sturman’s documentary, an elderly Chinese man weeps as he recalls trying to escape with his family. His mother was stabbed as she nursed his baby brother in an attempt to quiet him; the infant was plucked by a bayonet and tossed aside like trash. For a while, both survived, the baby crawling over bodies toward his mother’s voice and drinking her milk while her wounds gushed blood. It wasn’t for long. “I thought, Mom just died,” the survivor tells the camera. “What should I do?

Nanking, which can’t by any stretch of the imagination be considered entertainment, is full of such stories, told from various perspectives—the tormented, the tormentors, and the saviors. Even the dead speak: In addition to interviewing witnesses, Nanking’s directors also brought in actors to personify men and women who were instrumental in maintaining the Safety Zone, using their letters and diaries. It’s gimmicky, but most of the time it works. Jürgen Prochnow, as sympathetic Nazi businessman John Rabe, and John Getz, as missionary George Fitch, who smuggled 16 mm footage of the war crimes (some of which we see here), channel ghosts quite naturally and, in Getz’s case, poetically, finding a rhythm in Fitch’s words. Even Woody Harrelson, arguably the most famous face here, disappears as he stoically speaks as surgeon Bob Wilson.

Mariel Hemingway, however, is considerably less successful in a role that’s crucial to the sliver of sunshine the filmmakers try to force in the closing minutes of Nanking. Hemingway portrays Minnie Vautrin, the American dean of a women’s college located within the Safety Zone. In time, the Chinese came to refer to her as the Goddess of Mercy: Vautrin risked her life to block the Japanese from harming her students, according to tearful commentary from survivors as well as Vautrin’s own journal entries. But Hemingway never fully embodies the heroine, never lubricates her dialogue beyond table-­reading stiffness.

It’s a significant failing, but the remainder of the film is far too powerful to be sunk by it. Nanking bombards you with words and images of acts too barbaric to fully absorb. Japanese soldiers casually describe rape techniques. Old women recall sacrificing themselves to save family members. While these people speak, you see a city destroyed, its citizens running in fear or piled on top of one another when their attempts to flee backfired. The last chapter of Nanking shows the area reborn, with soaring music playing as we’re reminded of the efforts of Vautrin, documentarians, and others who put themselves at risk to not only save the Chinese but tell the world the full story. It’s here, and it’s unflinching.