Uncovering Unit 731's History Requires Strong International Cooperation

More than 80 years have passed, and we are only now beginning to realise how much Japan has yet to disclose, according to Lim Shao Bin, a Singaporean historian who has spent decades tracing Japan’s biological warfare activities in Southeast Asia. In a recent interview with Xinhua, Lim stated that as each new discovery reveals how much remains undocumented, filling these gaps will require broader international cooperation among governments and academics, both to advance cross-disciplinary research and to encourage the release of more historical records.

Since the 1980s, Lim has collected wartime Japanese documents from second-hand bookshops and government archives in Japan. Over the past decade, his research has focused on Unit 9420, a lesser-known biological warfare unit of the Imperial Japanese Army that operated in Southeast Asia. Unit 9420 was established as part of Japan’s wider biological warfare network, with personnel recruited from established units, including the infamous Unit 731 in Harbin, as well as other units in Nanjing and Japan itself.

According to Lim, Unit 9420 built an entire production line for germ bombs in Singapore and what is now Malaysia. Vegetables were cultivated to feed rats, which were bred to host fleas. The fleas were then infected with plague bacteria, placed inside glass bombs, and shipped to the battlefield. Headquartered in Singapore, the unit had branches across present-day Malaysia, Indonesia, the Philippines, Thailand, Vietnam, and Myanmar. “The Japanese military came to Southeast Asia because the climate was ideal,” Lim said, noting that Japanese researchers concluded fleas bred most effectively at temperatures between 27 and 30 degrees Celsius with humidity levels around 90 percent. Many of these findings are published in the 2025 book “Oka 9420 Unit, Japanese South Army BW Troop,” co-authored by Lim and Chinese scholar Wang Xuan.

New evidence continues to emerge. One of the most significant is recently uncovered testimony from the British National Archives by a former prisoner of war identified as W. B. Williams, which may provide new clues that British prisoners were also subjected to human experimentation. Captured in Singapore in February 1942, Williams wrote that he and many other prisoners were vaccinated and inoculated. Blood samples were taken, and “a glass tube was inserted into the anal passage.” Within days, he recalled, a large number of prisoners, including himself, experienced severe headaches, diarrhoea, dysentery, and abdominal symptoms. According to his testimony, similar procedures were repeated three times over three years at different camps, each time followed by the same symptoms. “If that was not experimentation, I do not know what else to call it,” Williams wrote. Lim noted that prior to this, no records had been found indicating that British nationals were also subjected to human experiments. However, the possibility was not entirely surprising to him. In his 2025 book, he had already questioned whether Unit 9420 conducted human experiments on British subjects, citing a June 1944 incident where 17 British prisoners of war in Singapore contracted scrub typhus, a disease not endemic to the area, and three died. A separate medical report compiled by Unit 9420 noted that military doctors had successfully isolated the pathogen from wild rats and transported it back to “Syonan-to,” the Japanese name for occupied Singapore. “The disease did not originate here,” Lim said. “It was brought here.”

Lim also highlighted another unexposed chapter. After Japan’s surrender, the United States sent several teams to investigate Unit 731. The unit reportedly handed over approximately 8,000 pathology specimens to the US authorities. However, these materials have never been disclosed to the public, and no legal proceedings regarding war accountability were ever pursued against the unit’s members. “International cooperation is crucial,” Lim stressed, noting that only through coordinated efforts by governments and research institutions can countries like Japan and the United States be persuaded to release additional archival documents. “If only individual researchers or universities request the disclosure of such information, it will be very difficult to make progress.”

For Lim, international cooperation extends beyond the release of documents. Understanding the remaining historical evidence increasingly requires expertise from various disciplines. He recalled discovering a 1942 inventory listing hundreds of chemicals transferred from Unit 731’s headquarters to Unit 9420 for a six-month period. Each entry was recorded using Japanese katakana characters. “Without a specialist in that field, it is almost impossible to interpret,” he said. “We need the involvement of various fields if we are to fully expose these historical crimes.” In June this year, Singaporean media channel Channel NewsAsia released a documentary titled “Inside Unit 731: Japan’s Secret Human Experiments.” Lim noted that many viewers in Singapore were shocked to learn of their country’s connection to Japan’s biological warfare programme, as many had thought Unit 731 operated only in China.