Almost every country in the world participated in World War II. Most were neutral at the beginning, but only a relatively few nations remained neutral to the end. The Second World War pitted two alliances against each other, the Axis powers and the Allied powers; the Soviet Union had 35 million serving soldiers, the U.S 16 million, Germany 13 million and Japan 6 million. With millions serving in other countries, an estimated 300 million soldiers saw combat.
The leading Axis powers were Nazi Germany, the Empire of Japan and the Kingdom of Italy; while the United Kingdom, the United States and the Soviet Union were the "Big Three" Allied powers.
During the 5 years conflict some countries have switched sides and some have been driven in the conflict by previous agrements like Republic of Korea which have been under Japonese rule and Libya under Italian rule.
Here is a map of all the participants.