If this truly were "extremely easy," why would you even need to bother asking us? This is in fact, a very intricate and complicated subject, and in 3-5 sentences, even lo-o-ong ones, you can only hope to scratch the surface, but here goes in 4 (LONG) sentences.
Nazism, fascism, and communism are forms of government; militarism is a tendency of a government to commit extraordinary resources to the military, and pre-war Japan had, as its form of government, an almost feudalistic, theistic monarchy. Inasmuch as all these systems of government, which can be lumped under the term, 'statism,' make the state supreme and the people subservient to it, they all place into the hands of a leader who is bent on expansionism, the tools to carry out his expansionist agenda without the possibility of effective interference from the populace. Such was the case for Hitler, Mussolini, Tojo, and Stalin, and while the first three of these leaders had formed alliances before the war, Hitler and Stalin entered into a non-aggression pact with regard to Poland. When both their nations invaded that country from opposite sides in September of 1939, in blatant violation of their treaty, and with Britain and France having agreed to support Polish independence, all the pieces were in place for the flurry of declarations of war that marked the beginning of WWII.