World War II had several causes, mainly having to do with the treatment of Germany after World War I when the country was forced to accede to the Treaty of Versailles. Many German nationals and government officials felt that the treaty unfairly placed all financial burden on Germany for war reparations and asked the country to cede lands it had formerly owned.
Because of the demands of the treaty, Germany had to print obscene amounts of money, leading to a state of hyperinflation. People could be seen taking entire wheelbarrows of paper currency to buy a loaf of bread. German currency, therefore, was worth less than the paper it was printed on. Unemployment ran rampant among the German populace, and Hitler took the opportunity to seize political power, stirring up discontent.
Germany gradually began making military expansion into its former territories to reannex ethnic Germans into the country. Although this was against the terms of the Treaty of Versailles, Britain and France said nothing. It was only when Germany invaded Poland in September of 1939 that they declared war on Germany.
Meanwhile, in Asia, Japan was struggling with poor natural resources and existing under the rule of Emperor Hirohito, a hardline military supporter. Japan's poor natural resources led it to stretch its territory into China and the Pacific Islands.