Why were militarists able to gain and exert so much power in Japan in the 1930’s? Did their control of government make the pacific phase of World War One inevitable?
One argument for why militarists became so powerful in Japan in this period is that the Japanese public had lost faith in government institutions. There was basically an inter-class struggle in Japanese society at this time between the higher, wealthier classes and the lower, poorer classes. Members and participants of militaristic groups at this time felt that military rule and totalitarianism would be better for the lower classes than capitalism. This general resentment among the Japanese public and particularly among the young military officers resulted in growing support for militarists.
I don't know if these developments made World War II (you can't mean WWI, WWI ended in 1918) inevitable but it is certain that they caused Japan to align with Germany and Italy in WWII. And since the Allied powers (Soviet Union, Britain, US) won World War II, this led to the consequent attacks on Japan in 1945. So we can say that militarism cost Japan much death and destruction.
It was not just top-down, though, Japanese culture had become increasingly militaristic since the late nineteenth century. Japanese leaders (and in this, I suppose it was top-down) had long admired the example of Prussia as an example of how a new, modern state could develop with militarism at its core. Japanese schools reinforced this, as did local authorities. Japan was a militaristic society, not just a government hijacked by a militaristic faction.
They were able to have so much power largely because of the constitution. The military could bring down any government it wanted by having the Army or Navy minister step down. They could simply refuse to appoint another until a government that was to their liking was formed. The Diet had no real control over the cabinet, but the military did. That's a recipe for a rise in militarism.
Japan was going through a transition at that time. They did not fully move to a democratic-like constitutional government until after World War II. At this time, nationalism and militarism took advantage of the instability of transition. The military often come into more power in situations like this.