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Initially, after WWI and into the 1920's, it was basically a two party representative democracy under the Meiji constitution. Voting rights were limited but suffrage improved in the 1920's. As unrest grew, including assassinations and attempted assassinations, the military assumed more and more control, eventually leading to militarized statism (aka fascism or national socialism) by the 1930's. Unlike Germany and Italy, the statism wasn't focused around a particular individual but around the military leadership as a whole.
For more see TaishΕ Period and Japanese Militarism