Before the victors set up The Nuremberg trials to try the Germans and the Tokyo trials to try the Japanese, there are no crimes known as war crimes or crimes against humanity or genocide.
Such crimes are a creation of the victors. That is why Napoleon and The Kaiser were never hanged.
A good book is A History of War Crimes by Bryant.
In the Spanish civil war, the Government received aid from the Soviet Union and volunteers from Europe and the United States. They were fighting the military which staged a coup assisted by Italy and Germany’s Hitler.
The volunteers included Americans like Ernest Hemingway. They were disgusted with the indiscriminate bombings of innocent civilians and the refusal of western powers and the US to prevent such bombings.
On the far side of the globe, China was also facing indiscriminate Japanese bombings of civilians alone. The western powers and the US again watch from the sidelines.
Individual Americans and Malayans volunteered to fight for China.
One Ipoh-born Captain Ho flew 18 combat missions in a B-25 Mitchell bomber over occupied China.
He is the last surviving member of the “Flying Tigers”, the legendary Chinese and American joint flying squadron, which fought against the Japanese in World War II.