World War II was the single largest conflict in human history, one that propelled the world into the modern age. The results of the conflict set-up the Cold War, introduced the nuclear age, began the development of the computer, and caused fundamental social changes that are familiar to citizens across the world. Less clear are the War's origins, which are complex and defy easy explanation, especially considering that the War was really several, interrelated wars. So what are the causes of WWII? They can be roughly divided into three categories: ideological, economic, and political.
The two main ideological drivers of World War II were nationalism and imperialism. In this sense, this is similar to the causes of World War I, and World War II has often been called a continuation of World War I. The two main belligerents of the war: Japan and Germany, both had a public mood and governments animated by extreme nationalism tinged with militarism, often known now as Fascism (the governments of Italy and Germany were both explicitly Fascist). The Japanese and German governments both felt their nations to have been slighted in the aftermath of World War I and wished to assert their power on the global stage and to have their nations acknowledged as important and to do so with their armed forces. Imperialism is the other important driver, especially for Japan. Japan's invasion of modern-day China began in 1931 and expanded their colonial empire, which already included Korea. The Japanese felt their nation was over-crowded, yet devoid of national resources, so in order to control their own destiny they felt they had to seize resource-rich areas to avoid being dominated by foreign powers. The Nazi German government under Adolf Hitler wished to expand eastward in order to secure oil and food resources for Germany.
Economic considerations played a powerful role in driving both Germany and Japan to war. Germany faced economic humiliation and disaster in the 1920's after being hamstrung by the reparations they were forced to pay in the Treaty of Versailles, which officially ended World War I. The Great Depression further racked Germany's economy and when Hitler came to power in 1933, Germany's massive rearmament and public works' programs effectively ended unemployment. The build-up for war saved Germany's economy and necessitated aggression in order to maintain the economic status quo. The war would have helped Germany's economy by creating satellite states and colonies that they could pillage for resources. Japan, as mentioned above, was concerned that they did not have the natural resources to maintain an industrial economy, especially after the Great Depression. Much of Japan's aggressive expansion was into areas which contained resources such as iron ore, oil, and rubber.
The last, and perhaps, main cause were political considerations. Specific political policies drove the nations of the world into conflict. Adolf Hitler's aggressive policy was designed to both seize territory and to provoke conflict with Germany's historical enemies of France and Poland, and inevitably led to a confrontation with Communist Soviet Russia. The policies of the Nazi's were in direct conflict with the democracies of Western Europe and America and the Communist government of the Soviet Union. Hitler's insistence on seizing German-speaking areas of European countries eventually provoked a war with France and Britain. Hitler's policy was also directly at odds with the restrictions placed on Germany by the victors of World War I, and was consistently provocative. In Japan, the desire to have the military supreme over all other aspects of the State crushed alternative views and led the country on a path to war in it's quest to be the dominant power in the Pacific Ocean and the world's preeminent naval power. The policy of continuing aggression against peaceful nations and against European colonial possessions eventually led the United States to embargo Japan, which led Japan to become very aggressive against the United States, eventually attacking the US Pacific Fleet and drawing the United States into the global conflict.
These, briefly, were the main causes of WWII. There were other consequential actions by nations involved in the War besides the two main belligerents and there were a host of events, social changes and attitudes, and ideological subtleties that brought nations to the point of cataclysmic conflict. This set of complicated, but familiar, engines of conflict happened to coalesce at a point where the whole social life of nations were industrialized and pitted against one another in battles of annihilation. This is why the Second World War had such a horrific death toll and caused so much change in such little time.