If one had to name a particular cause of World War II; it would be the Treaty of Versailles that ended World War I. Germany was treated most unfairly by the terms of the treaty. Germany was forced to accept full responsibility for the cause of the war, and full financial responsibility for its costs. This caused so much bitterness and resentment among the German people that it provided a ready platform for Adolf Hitler. Hitler railed about the unfairness of the Treaty, and also the need of the German people for Liebensraum, or "breathing space." This was the basis for his marching into the Rhineland, and his taking of all of Czechoslovakia when he had been promised the Sudetenland. This should have been sufficient warning to those who opposed him that he would not keep his word, but they were more inclined to appease him. Finally, on September 1, 1939, he invaded Poland, and the war in Europe began.
Strangely enough, the war in Asia with Japan was also largely the result of the Treaty. Japan had entered World War I on the side of the Allies hoping to gain large amounts of German held territory in Asia, but walked away empty handed. This played into the hands of the militant war party in Japan who started a campaign to take Manchuria and large areas of Indochina. When the United States stopped shipments of oil and scrap metal to Japan, the Japanese high command saw war with the United States as inevitable. Adm. Isokuru Yamamoto, who commanded the fleet that attacked Pearl Harbor, had reasoned that the United States did not have a "warrior heritage," and did not have the stomach for a long war. He believed that if the Pacific Fleet could be eliminated, the United States would make peace before the Atlantic Fleet could be repositioned. It was for this reason that Japan attacked Pearl Harbor on December 7, 1941.
Japan had previously signed the Tripartite Pact with Italy and Germany as a result of which, when the United States declared war on Japan, Italy and Germany declared war on the United States within 72 hours. And the full war was on.