Short answer is imperialism.

Slightly longer answer: By the 1930s, Japan was in the final stages of propping up an unsustainable business model.

Observe below, the mix of defense spending as a

proportion of overall government spending. In the relatively peaceful years of 1930s, the spending barely came under 30%. That is a massively high mix for a developing country.

Since the Meiji restoration, Japan has had one goal, that is to join the ranks of industrialized and imperialistic world powers, and not become anyone's colony. It has embarked on an all-hands-on-deck program of modernization and militarization. This kicked off a vicious cycle of getting into debt and winning territories to finance spending.

Arms race and conflict >> debt >> need money and resources >> conquer and colonize >> further tension with neighbors and arms race >> rinse and repeat

1. Destabilize and usurp Korea
2. Fight a war with China to take over Taiwan
3. Fight against Russia to take Manchuria
4. Jump on the WW1 bandwagon and get key holding in the Southern Pacific from Germany
5. Destabilize China to take over the whole country ex Manchuria
6. Jump on the German invasion of France and get Indochina from France
7. Next up! Dutch colonies and Oil

By the time Pearl Harbor happened, Japan has single handedly financed a vast military that defended all territories between today's Vietnam, Mongolia and Guam, with a GDP that was around half of Germany. Even if it was not technically bankrupt, structurally, it was. An oil embargo by the US was enough to send Japan's broken finances over the edge and have the country commit strategic suicide that was the Pearl Harbor attack.

I guess Japan needed someone with enough vision and foresight to say "STOP! This isn't working!" But the tricky thing is, for all the Japanese people at the time, it felt like great success - winning war after war, and the empire rapidly expanding. Media fanned the flames and the people DEMANDED expansion. Democratic inertia is a hard thing to kill.

With benefit of hindsight, the Yoshida doctrine has always been the right answer. If Japan was to be an industrial power, with vast resource needs, it was always structurally impossible to finance the defense of it's supply chain through it's own military. PM Yoshida decided that Japan should defend it's supply lines by outsourcing the defense of it to the US, and work through the UN to promote stability in the region. The rest, as they say, is history. Japan today has military spending under 1% of GDP and 5% of government spending - and has the third largest GDP in the world.

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