<human>:  Why did the Japanese win the Battle of Na San?
<bot>: The Japanese were able to win because they had a larger number of trained soldiers. The Japanese army had more than 1.3 million soldiers, while the Chinese army had less than a million. The Chinese army was not well trained, and many soldiers were not even soldiers but farmers and students. The Chinese army also had a lot of old soldiers who had been in the army for a long time and were tired of fighting.