World War 2 was fought between two groups of countries.
On one side were the Axis Powers, including Germany, Italy and Japan. On the other side were the Allies. They included Britain, France, Australia, Canada, New Zealand, India, the Soviet Union, China and the United States of America.
Among the causes of World War II were Italian fascism in the 1920s, Japanese militarism and invasions of China in the 1930s, and especially the political takeover in 1933 of Germany by Hitler.