Why was China so badly beaten by Japan during World War II?

The combat effectiveness at that time was not as good as that of Little Japan.

Little Japan is not fighting abroad.

At that time, China seemed to be a unified big country, but in fact it was divided by warlords and one side was fighting for its own interests regardless of the overall situation.

In 1931, Little Japan invaded Northeast China. Due to Chiang Kai-shek's non-resistance policy, Northeast China was handed over to Little Japan, which made the Northeast a rear base for Little Japan during the all-out war of aggression against China after 1937. If Japan's mainland had been relied upon solely to supply the invading army, Japan's army would have starved to death long ago.

Even a large army is of no use if the country is poor and the soldiers are weak.