Japan did sign the Geneva Convention but, like the USSR, failed to ratify it, so was not bound by the laws. However, in 1942 Japan made a promise to abide by its terms and indicated it would observe the Hague Convention of 1907.
While the extent of the atrocities committed are still a matter for intense debate, there is little doubt the Japanese grossly violated the Geneva Conventions during the Second World War. The very same year they had agreed to stick to the rules, Japanese forces savagely brutalised thousands of American and Philippine POWs on the infamous Bataan Death March, killing more than 5,000 men through starvation, beatings and execution.
Inconceivably to many, such cruelty is explained by the Japanese military’s firm belief that surrender was the ultimate shame and dishonour; for them, POWs did not deserve humane treatment. Following the horrendous civilian slaughter witnessed in the Second World War, a revised Geneva Convention was drawn up in 1949 to address the treatment of non-combatants.
It also included the prohibition of scientific experiments on POWs in response to the torture exacted on prisoners by German and Japanese doctors. Japan wasn’t among the original signatories in 1949, but it became the 24th state to ratify the Geneva Conventions on 21 April 1953.