China had been in an on-and-off civil war since 1924, which grew in intensity as the Communist Party gained power in the 1930s. Consequently the country was a complete wreck with no effective governance outside of the major cities for this reason - in today's terms, a failed state - before the Japanese even invaded.

As such, most of the deaths were due to starvation, exposure, low-level violence and other dangers that arise in a highly unstable country. Add to that the fact that the north of the country is prone to natural disasters - floods, droughts, famines - and these latent factors rather than actual violence explain much of the death toll

Indiscriminate violence against civilians by all sides (Communist, Nationalist and Japanese) explains most the rest. Japanese war crimes are the best known but far from the only significant cases - the Nationalists killed hundreds of thousands of civilians when they broke the dams on the Yellow River.