Which neighbours could theoretically challenge their might?
China had just been through civil war and still had a lot of tumult going on.
Most of the other Asian countries were British colonies – India, Indonesia, Pakistan, Bangladesh etc.
Japan was next highest in population, had an educated workforce, and a militaristic leadership that wanted to become more powerful and join the European countries as a colonial power.
In fact, Japan not being a powerful military would have been more surprising.