Strategically, Japan is a very fragil country, large part of this
nation's wealth and population is concentrated around Tokyo
metropolitan area,which make Japan very vulnerable to aerial attack,
not to mention nuclear attack. And also remember that Japan has to
import almost all the natural resources it needs,which means naval
blockade will be devastating to japanese economy. 19 centry's Japan
managed to become a big military power only because she took
advantage of
power vaccum in east asia and other struggle among big powers. Look at
east Asia's map in late 19 century: China's minority dominated Qing
Dynasty was internally very weak, and Qing's rules had to spend most
of their time dealing with peasant rebellions, at the
same time,keep a eye on the majority Han. America could get most of
what she need from its own hemisphere, so she didnt show much interest
in Asia, then dominant great power,namely,the British Empire,
was the most influential power in EA, but she had to constantly fend
off challenges from Russa, who shared long-running border with China.
Japan ascent to Great Power status was achieved largely through its
alliance with Brition, because of its alliance, Japan was able to
defeat China and Russia,and in the process of it, occupied Korea,Taiwan
and made Manchuria a semi-colony. Only after that, Japan was able to
claim its status as EA's dominant power, even so,
in the late 30s of last century, the fast-uniting-china held real
possibility of suppass Japan, which forced Japan to do whatever it took
to stop China's unification,including direct invasion of China,sadly
for Japan,she could never be able to occupy a country as big as china
,she just didnot have the resources. The adverture in china depleted
japanese national resources and made japan scramble to find new
resources,in the end, forced japan to even bigger adverture,Pearl
Harbor,which spell the end of Japan position as military power.

Japan could never be able to be asia's police man as long as china was
united, Japan was simply too small and resource-poor,which mean Japan
could not survive in a real war with his giant neighbor,no
matter how richer Japan is than China.