If one had to designate a particular cause for World War II, it would havve to be the failure of the Allied Powers to correctly resolve the issues raised by World War One. The Treaty of Versailles which settled war issues imposed harsh terms on Germany, including the loss of Alsace-Lorraine, a war guilt clause, and exorbitant reparations. This caused tremendous resentment among the German people, and provided a platform for Adolf Hitler who appalled to German nationalism and railed against the unfairness of the Versailles treaty. Hitler's efforts to gain additional territory for Germany led to the war when he invaded Poland.
In the meantime, the Japanese had left Versailles disappointed, as they had hoped to gain German territory in Asia. They received nothing, and as a result, a militant group took over the Japanese government. Intent on building an Empire, the Japanese invaded Manchuria and began building an Asia Empire. Their motto was "Asia for Asians." It was Japanese aggression in Asia which led President Franklin Roosevelt to embargo sales of oil and scrap metal to Japan, which was followed shortly thereafter by the Japanese bombing of Pearl Harbor.
The Tri-Partite pace signed between Germany, Italy and Japan led the first two to declare war on the United States shortly after the U.S. declared war on Japan.