This is for World War II
After World War I, the collapse of American stock market, then Europe, and quickly spread worldwide, especially Germany who had to take responsibility for the warfares. It created a worldwide economic collapse.
The cause of WWII:
When Japan was seriously in need of industrial materials, the while country was destroyed. They declared war to get resources from other colonies in Asia where most colonies belongs to Great Britain, France, America, and Netherlands.
Japan needed allies so they teamed up with Germany and Italy, called the Axis Power with the intention of taking over the world.