In the 1930s, Japan's notorious Unit 731 carried out brutal experiments on the population of recently-invaded Manchuria. Whole villages and towns were deliberately infected with plague, and sufferers were dissected alive.
Prisoners of war were shot and operated on without anaesthetic so army doctors could practise field surgery. In World War II Germany, concentration camp doctors like Josef Mengele selected twins and Romany prisoners for obscure medical experiments, and killed enemy prisoners in low temperature or high altitude tests, supposedly to protect their own sailors and airmen.
After the war, many Nazi 'Doctors of Death' were brought to justice. But in Japan the head of Unit 731 cut a deal with US intelligence; the Americans knew they could never replicate biological data gained through experiments on humans. There are suggestions the US used Japanese bio-weapons in the Korean War -- but America began to suspect the North Koreans had their own unorthodox methods: brainwashing US prisoners with drugs.