It's been quite a while since I've segued into one of these discussions, but based on what I've seen so far I'll throw a couple things out there.
There's been a good deal of discussion regarding the roots and evolution of militarism in Japan, but less discussion of the roots and evolution of democratic institutions. By and large, Japan's history from the Meiji Restoration to the Mukden Incident was oriented towards autocracy in one form or another with their democratic shift in the 1920's being rather exceptional. It bears considering what factors contributed to this deviation from an otherwise consistently militaristic course.
The environment from which Taisho democracy sprung was created by the convergence of three main influences. The Genro oligarchs which had dominated elite society since the later nineteenth century were reaching the age where they were either retiring from public life or just dying off. This also conveniently coincided with the death of Meiji emperor and the succession of the Taisho emperor. The Meiji emperor had been fairly marginalized politically by the Genro but his son was even less inclined to flex his imperial muscle. Some attribute this to persistent mental illness, but regardless of your interpretation of his behavior the Taisho emperor was not a very strong personality and had little interest in governance. This allowed for an unprecedented power vacuum into which the ambitious and opportunistic were suddenly allowed access. The character of these new leaders was shaped by the broader era in which they were living. These were the years just following World War I, and the seeming vindication of self-determination and popular will this implied was still impacting the ideology of the intelligentsia. Japan had sided with Great Britain over Germany in the war at least partly through opportunism, but also because Japan wanted to cultivate the image of being a responsible and progressive world power. Sincere or no, Japan's new democrats took the idea to heart. Civil society was the buzzword of the era, with political associations of every shape and size popping up like mushrooms in most of the Western world, and Japan followed suit.
You can have too much of a good thing though. This sudden massive proliferation of different activist groups hindered efforts at coalition building and decisive policy making. Moreover, the intelligentsia of the military officer corps was also politicizing; gaining an awareness of its own ideology and agency. The notion that Japan should act as a counter-balance against Western imperialism in Asia and even liberator of exploited colonies had already existed, and honestly began in sincerity. This train of thought converged though with another notion rapidly becoming dogma amongst Japan's military thinkers. In the wake of the global economic collapse entering the 30's it was believed that Japan had erred in over-reliance on foreign trade to sustain itself. In this view, Japan had to secure its own resources to ensure its survival. The most direct method of achieving this was the tried and true model of a Western-style colonial empire, which dovetailed well with Japan's earlier expansions and conveniently aligned with lip service to Asian liberation ideology. In this the idea of the Co-Prosperity Sphere was formed.
On the domestic front, the inefficient if well-meaning Taisho democrats had taken the brunt of popular backlash for Japan's economic woes in the new decade. Confidence in the emperor was not very high at the time either. To this end, military-inspired propaganda began agitating for a so-called "Showa Restoration," which would install a new emperor enjoying the confidence of both the people and the military. This was ultimately successful, and while the Showa emperor was not significantly more willful than this predecessor he served as an excellent symbol for change and was at least nominally sympathetic towards the military establishment. This was useful considering that the constitution at the time, written in the Meiji era, placed the military accountable only to the emperor, rather than to any civilian authority. The commander of the armed forces and the Prime Minister of the Diet were effectively of equal political clout and strength and the government really had no directive authority over the army. The IJA initiated their invasion of Manchuria without the Diet even knowing about it until it was over. Under the constitution the military didn't need to consult with or explain themselves to anyone. Even the emperor tended to approve the army's policies after they'd already been executed.
To avert this you really need to effect a change in Japan's political culture. The military's agency must be reduced, either collectively through the constitution as written or individually through the political awakening of the officer corps. Both are problematic.
