Why does the world war II happen?
More specifically, economic instability/the loose ends left by WWI led to militarism.
After WWI, Germany was subjected to heavy reparations payments and other measures which crippled its economy. There were also a lot of political changes. Relatively quickly, the Great Depression came along, which only worsened the issues. Then, along came Hitler, a populist who scapegoated several groups (Jews, Eastern Europeans, liberals, Communists, socialists, leftists, etc.) and took every opportunity to talk about how the Germans had been betrayed, the Germans were the superior race, the Germans needed to take what was theirs (i.e. Europe, in a concept known as lebensraum), and so on.
Italy did not get what it wanted in the Treaty of Versailles following WWI and had a lot of economic troubles, just like Germany, which led to a populist leader, Mussolini, taking power, who, much like Hitler, blamed groups for the current instability and urged people to militarize and recreate the Roman Empire. One of the central tenets of fascism is military conquest for the glory of the state, actually.
Again, in Japan, economic instability made the people jittery. And Japan's existence has been a balancing act of maintaining Japanese culture while conforming to Western culture since the early 1800s. In order to secure that and to respond to the economic crisis, militarism slowly swept over the Japanese government until Hideki Tojo, former Minister of War, was appointed Prime Minister a bit more than a year before Pearl Harbor.