This is a really complex question, but I'll try and boil down the main points.

After the end of the Taisho period in 1926, a sort of militarist fever took place among many military field officers (majors and colonels and that sort of thing). This was a revival of the medieval Japanese idea, Gekokujo, or "The weak rules the strong." The idea was that young idealistic officers could and should be able to make decisions over the slow, doddering, unambitious old men who were in power. Various incidents perpetrated by these fanatics began to show a clear trend in Japanese society. Various politicians and businessmen would be assassinated by military hotheads. In 1931, a Japanese colonel unilaterally invaded Manchuria without consulting his superiors or the government. In 1932, Prime Minister Inukai was assassinated, but his assassins got off lightly due to popular approval.

This all came to a head in 1936 during the 2-26 incident. A group of Imperial Japanese Army fanatics decided that they were sick of the parliamentary system and sought to execute a "Showa Restoration" in line with the Meiji Restoration. The idea would be that they would bring down the civilian government and reform it so that the Emperor would have more centralized power, then introducing institutions that the hotheads approved. The coup failed spectacularly, and an incensed Hirohito gave wide-reaching authorization to punish the army faction responsible. However, the civilian government collapsed, as they were unable to do anything in light of the coup. When they attempted to reform the civilian government, the Army Minister disapproved of some of the cabinet choices, and the Prime Minister acquiesced. Further military demands into the cabinet-namely, that a government must have an active-duty officer as the Army Minister-gave the military wide power over controlling a civilian government. Any civilian government that did not do what the army or navy wanted could be immediately brought down by any army or navy officer refusing to serve on the government.

Now, the Imperial Japanese Army was theoretically controlled by the Army Ministry. However, the armies in Manchuria-known as the Kwantung Army-were essentially autonomous from the rest of the Japanese government. The head of the Kwantung Army at the time was Hideki Tojo. He had a group of secret police known as the Kempeitai that would crack down on dissension in his essentially private fiefdom in Manchuria. His main opponents in the military-one of whom, Kenji Ishiwara, was the above colonel that unilaterally invaded Manchuria-were essentially sidelined, and he eventually became Prime Minister in 1940.