Japan is … odd.
Most countries have a very clear chunk of reasonably good land that serves as home to a specific ethnicity. That group forms a government to serve the needs of those people in that place, and then that government steadily expands its writ over more territory and peoples. The valleys Nile, Thames, Ganges and Argun for the Egyptians, English, Indians and Chechens; Muscovy for the Russians; the Beauce for French, the Zagros Mountains for Persians, the Tibetans on their namesake plateau, and so on.
Japan doesn’t really have something like that. The Japanese islands are so steep and arable land so hard to come by that even as late as early 1800s, well over a millennium after after of the emergence of the Japanese ethnicity, the Japanese still lacked a common government.
So…what made Japan matter?
First, isolation. In the imperial age Japan was beyond the back of beyond. Between its island nature and its position on the northeastern corner of the Afro-Eurasian continent system, no one simply happened by. Anyone who wanted to reach the Japanese really had to want to reach the Japanese. If the Japanese’s geography wasn’t as good at isolating them from the rest of the world as one another, they would have been conquered ages ago and never emerged as a people of consequence.
Second, industrialization. What Japan could not do with muscle and wood and arrows and horses and manure they could do with steam and gunpowder and rifles and electricity and chemical fertilizers. The industrial suite of technologies enabled the late-19th century Japanese to overcome their horrid internal geography and forge the truly unified economic and political space we know today as Japan.
But that was hardly the end of the story. More the beginning. Because aside from its people, Japan has nothing that enables industrialization. Steel foundries require high quality iron ore, and Japan has none. Power lines require copper ore or bauxite, and Japan has none. Electricity requires coal or uranium or natural gas, and Japan has none. (Japan’s solar and wind potential for greentech energy is similarly pathetic.) Name an industrial input. Japan doesn’t have it.
And so, the only way Japan could industrialize – the only way Japan could reliably unify – was to raise an empire that could funnel the various inputs of the modern world to the Home Islands. To exist in the modern age, Japan had no choice but to expand into empire.
The Japanese know this in their bones, and they know the converse is true as well. The Japanese fought so hard in World War II less out of nationalism or because their emperor ordered them to, and more because they knew failure would mean a return to mutually-warring Balkanized medievalism.
The modern Japanese also know something else in their bones. No matter how powerful they become, no matter how well they anticipate and administer and execute, no matter how potent their navy, they will never be able to challenge the United States. Attempting to do so pisses the Americans off, and that has consequences. Dire, horrific, searing consequences.
With their WWII defeat, the Japanese prepared themselves to vanish from history. Unity required industry required empire, and their attempts at empire had failed.