Why did World War 2 start?

World War II officially started on September 1, 1939 when Hitler invaded Poland. Britain and France gave Hitler an ultimatum to leave the country, but he refused. Britain and France then declared war on Germany and its ally, Italy, two days later. The roots of the war go back much further than that, however.
World War II started because World War I was not truly resolved. In the Treaty of Versailles, Germany was blamed for starting World War I and had to pay an indemnity to France all while losing valuable industrial regions. The ensuing global depression of the late 1920s and early 1930s did not help as Germany spiraled into hyperinflation and extreme poverty. Adolf Hitler promised to make Germany great again and to avenge the wrongs of the Treaty of Versailles. Many Germans backed him and allowed him to annex Austria and Czechoslovakia. Britain and France, severely weakened by the war, followed a policy of appeasement because they were willing to do almost anything to avoid another war.
Italy joined the entente late in World War I because it expected to gain land from Austria-Hungary. When this did not materialize, Italy looked to one-time newspaper editor Benito Mussolini as its political strongman. He soon joined forces with Hitler and sought to recreate the old Roman Empire. His first moves were taking Libya and Ethiopia.
Japan was hit hard by the global depression and it felt disrespected by the United States in that the United States would not accept racial equality in the Treaty of Versailles.  A group of Japanese warlords took control of the country, and they sought to create an Asia that would remove the American and European imperialists and place themselves as the leading power in Asia. Japan's first move was to attack Manchuria and establish a puppet state there in 1931. Japan then expanded into the Pacific and made further inroads into China. Japan attacked China in order to obtain resources and potential slaves for further expansion across the Pacific. Japan allied itself with Hitler and Mussolini in order to form the Axis Powers in 1940.