The most important thing to know about Japan as a world power is that it's an anomaly. This was completely unprecedented, and this type of rise hasn't been replicated in any other state. At this time, Japan has no natural resources, no large-scale cultural capital, no major economic power on a wider scale. Then, suddenly, starting in the mid-19th century, it industrializes at an amazing rate, becomes a first-rate world power, and would go on to become one of the foremost military and economic powerhouses before falling into a gradual decline (but still very dominant position) within the last twenty years.

The lynchpin of understanding this huge jump (and the resulting position Japan takes on the world stage, which leads to everybody's fascination with it) is how it gets there in the first place - and remember, it has absolutely no physical advantages here.

The argument I'm making is: The policies pursued by the Tokugawa shogunate led to the social development of a national identity in Japan, which gave it the local social capital it needed to industrialize so quickly. This national identity started as a method to constantly weaken daimyo through bureaucracy, and as a side-effect, developed the first large-scale urbanized, centralized, constitutional-monarchy styled polity. This was a ploy for stability, first and foremost. Remember that.

Japan goes into isolation near the end of the Sengoku period - this is driven by the Spanish conquest of the Phillipines. Westerners are crazy motherfuckers who will invade and conquer you if you trust them is the logic, and this is very important, because it's this isolation from the world right before the start of the Renaissance that leads Japan down this roundabout path towards modern society (which gets some of it down right away, but at the expense of never actually getting there without major upheaval). The irony is that, following the above main point I've made is that without isolation, Japan never would have had the stability it needed to further develop this national cohesion.

Japan at this point was a proto-fascist polity. It was, for all intents and purposes, an invented state. The imperial court held no power for hundreds of years, and the actual throne had lost its power eras ago. The concept of a noble caste of warrior-monk samurai was an invented concept. Bushido never existed in the popularly known form pre-Edo period. The samurai never had connotations of religious piety or honor until the case of the 47 Ronin.

In what bad internet comedians make jokes about all the time, the fictionalized version of their story became mapped onto the popular consciousness regarding old-time samurai. That is, the Hollywood version of the truth became the official version of the truth. By this point, the samurai had become your standard nobility, with the very important distinction that as a warrior caste coupled with the shogunate's policies (and proto-concept, even) of monopoly on force (which, remember, wouldn't exist in its modern form til after Westphalia).

This sets the stage for the throne's first actual claim to power in Japan for ...pretty much the entirety of its existence as a Sinocultural entity. It was this invented idea that samurai were loyal warrior monks sworn to protect the emperor that caused the bakamatsu and Boshin War - this idea that the throne was supposed to have power that led daimyo disempowered through centuries-old realpolitik and upstart young samurai (read: upper middle class armchair revolutionaries) to believe that they had some duty to liberate the throne from the shogunate and would prescribe the throne powers it never had in the new constitution, which, coupled with this idea of a national identity, urbanized population, centralized bureaucracy, and monopoly on the legitimacy of violence that allowed Japan to industrialize at the pace it did and led to its nationalist-militarist-corporatist development that would set the stage for the postwar Japan that weeaboos cream their pants over.

Americans just stripped the nationalist-militarist sentiment for the most part, but more on that in another rant.

The only other real option is that Japan does not go into isolation - that's the only possible point of divergence for Japan as an entity. This, and if you wanna push me for it, I'll write up another rant on where that would go - would result in the end of anything resembling a unified Japan. Modern Japan would not exist without isolation.

tl;dr
•Japan, as a concept of a state that rose from humble beginnings to world power, is an anomaly.
1. Japan had no natural resources, no cultural influence over its neighbors, and no real economic power or leverage.
1a.Japan's only strength lied in the development of national identity before national identity was a thing.
1aI. The Yamato Dynasty's unbroken lineage, and attempts by all actual holders of power to maintain the legacy of that unbroken lineage, contributed greatly to the development of this national identity.
1b. Hideyoshi's [whose rule was not legitimized by the Emperor] edict of isolation sets the stage for the development of a proto-fascist centralized government.
1c. Ieyasu's [whose rule was legitimized by the Emperor] policies as shogun cements the throne's legitimacy at the time when it was weakest, and develops an urban population, highly developed highway network, and the beginnings of a national identity independent of servitude to one's lord.
1cI. Because irony is funny, it's that identity independent of servitude which developed an identity of servitude even stronger than had ever actually existed, which would also play a large role in the Japanese national psyche.

•Therefore Japan would not have developed that one distinct edge had it not undergone isolation.

Closing notes: Japanese history is pretty much just the backstory of 40k.