* United States: The United States entered World War II in December 1941, after the attack on Pearl Harbor by the Japanese. The US was the leading economic and military power in the world, and its entry into the war turned the tide in favor of the Allies.
* United Kingdom: The United Kingdom was the first country to declare war on Germany in 1939. The UK fought the war in Europe, North Africa, and the Pacific. It was a key ally of the United States, and its leaders, Winston Churchill and Clement Attlee, played a vital role in shaping the course of the war.
* Soviet Union: The Soviet Union was the largest country in the world, and it suffered the most casualties in World War II. The Soviet Union fought the war on the Eastern Front against Germany, and its victory at the Battle of Stalingrad in 1943 was a turning point in the war.
* China: China was the most populous country in the world, and it fought the war against Japan for eight years. The Chinese Nationalist Party, led by Chiang Kai-shek, fought the Japanese in China, while the Chinese Communist Party, led by Mao Zedong, fought the Japanese in guerrilla warfare.
* France: France was one of the most powerful countries in Europe before World War II, but it was defeated by Germany in 1940. The French government went into exile in London, and the French Resistance movement fought the Germans in France.
* Canada: Canada was a major contributor to the Allied war effort. Canadian troops fought in Europe, North Africa, and the Pacific. Canada also provided food, supplies, and munitions to the Allies.
Axis Powers
* Germany: Germany was the most powerful country in the world at the start of World War II. The German Wehrmacht was the most powerful military force in the world, and it conquered much of Europe and North Africa.
* Italy: Italy was a key ally of Germany. The Italian army fought in Europe, North Africa, and the Mediterranean Sea. Italy also provided air support for the German Luftwaffe.
* Japan: Japan was the most powerful country in Asia. The Japanese Imperial Army and Navy conquered much of East Asia and the Pacific. Japan was a key ally of Germany, and it fought the United States in the Pacific War.