Why Japan? It is bunch of crowded islands with absolutely no natural resources. It is also the reason why Japan always wanted to invade continental Asia. However, it is exactly because Japan lacks resources that compels the Japanese nation to adopt an offensively realistic but very opportunistic maritime imperialist foreign policy. Tokyo also realises that it is not a top-notch power, so it has long tried to align with the strongest power in the world order (British Empire from Meiji era to right before WWII, and then the US after WWII) in order conquer other lesser powers (Korea, Qing, ROC, Russia, USSR, etc.). Japanese elites also realises that striking Pearl Harbour was a strategic mistake, but not invading the much weaker China.
In general, Japan is highly opportunistic and has long mastered realpolitik as a regional power.
Internally, Japanese ideology is actually extremely feudalistic to its core due to the population's continuing perception of the emperor and respect for paternalistic social order as the ultimate guarantor of the national identity/existence.
As with today and exemplified by the US-Japan alliance, they most effective way to keep Japan down is to preserve its independence (especially keeping its emperor) but coerce Japan through both institutional and power means to give up part of its sovereign defence and foreign policy.
