TOKYO – Japan’s amnesia over its militarist past is being challenged by a compelling documentary film that suggests the Japanese army breached the terms of surrender in 1945 by leaving soldiers to fight on in China.

“The Ants,” to be released next month, records the struggle of the Japanese veteran Waichi Okumura to put atrocities on record and to tell the story of the forgotten soldiers left behind in China. Now 81, Mr. Okumura revisited Shanxi province where he fought, including a pilgrimage to the place where he and other recruits were “toughened up” by being made to kill Chinese prisoners with bayonets.

The story is remarkable because most Japanese veterans downplay atrocities and romanticize the war, whereas Mr. Okumura asks openly: “What the hell were we fighting for?”

Veterans who speak out typically have been ostracized by their comrades, but Mr. Okumura is supported by a dwindling group of fellow soldiers. The documentary covers their long and unsuccessful legal battle against the Tokyo government to show that 2,600 Japanese troops were made to fight alongside the Chinese nationalist warlord Yan Xishan until 1948, in clear breach of Japan’s unconditional surrender.

The Japanese government says the men volunteered to join the Chinese nationalists, leaving their units without permission. But the men say officers had quotas of volunteers to fill from each unit and that being told to volunteer by an officer was the same as an order to a Japanese soldier.

They say their mission was to maintain a Japanese military presence so that one day Japan could resume its territorial ambitions on the mainland. Mr. Okumura, who was held in China until 1954, said 550 of the men were killed, and he was among more than 700 captured by communist troops.

Testimony from survivors of both sides supports the case that there was a secret deal between Yan and Japan’s General Raishiro Sumida. Records show the men continued to be bound by Japanese army regulations, suggesting that they fought as soldiers of Japan, not volunteers in a Chinese army.

Japanese society is famed for its avoidance of confrontation. Mr. Okumura takes the opposite approach. He asks a Chinese victim of a brutal gang rape by Japanese soldiers to retell her ordeal for the camera, and recalls how he had kept lookout while fellow soldiers committed rape. At Tokyo’s controversial Yasukuni Shrine, where Japan’s war dead are commemorated, Mr. Okumura embarrasses young people by asking them why they don’t know their history and why they are worshipping dead soldiers.

A crowd of unreformed nationalists at the shrine cheers a speech by one of their heroes, the soldier Hiroo Onoda, who hid in the Philippine jungles for 31 years, unaware that Japan had lost. Mr. Okumura confronts him by asking, “Are you glorifying our war of aggression?” In perhaps the most shocking scene in the film, Mr. Okumura shows his friend Den Kaneko written testimony he found in China that records how Mr. Kaneko killed an innocent Chinese peasant by bludgeoning his head with a rock.

Mr. Kaneko, a tiny man who cares for his paralyzed wife at home, admits it must have happened but does not remember the incident. “It’s strange that I don’t remember, but killing people happened every day back then. It was nothing to me. I really was a demon,” he says.