Two groups of countries were involved in World War II. One group, the Axis Powers, consisted of Germany, Japan and Italy. The group on the other side, the Allies, consisted of the United States, China, Britain, France, Australia, Canada, India, New Zealand and the Soviet Union.
Because so many countries and regions were involved in the war, World War II is considered a global war. Many countries, including Sweden, Switzerland, Ireland and Spain, remained neutral during the war and barely got involved. Though many complex conflicts and incidents built up in the years prior to the war, World War II took place between 1939 and 1945.
On Sept. 1, 1939, Nazi Germany, led by Adolf Hitler, invaded Poland with the intention to continue to take control of Europe. This caused Britain and France to declare war on Germany. Across the world, Japan was also seeking to control most of the Asian continent and the Pacific region. In 1937, Japan battled China in what is now known as the Marco Polo Bridge Incident.
As the conflict spread, more and more countries began to join the war. By the summer of 1940, Germany had seized France, Belgium, Norway, Holland and Denmark. In 1941 Germany attacked the Soviet Union, which initiated that country's involvement. That same year, the Japanese attacked Pearl Harbor in Hawaii, which caused the United States to join forces with the other Allied countries.
The war ended in Europe when Germany's arms decreased and huge numbers of Allied tanks swept back across the continent and took control away from Germany. In 1945, the United States dropped atomic bombs in Japan to end the war in the Pacific. Over the war, approximately 60 million people died.