Top Photo:  From left to right: Joseph Stalin, Franklin D. Roosevelt, and Winston Churchill on the portico of the Soviet Embassy during the Tehran Conference. November 28, 1943. U.S. Signal Corps photo.

World War II was the most destructive conflict in human history and claimed the lives of millions of people all over the world. It was a complicated war that included entire countries being occupied, shifting alliances, and the redrawing of national borders.  

Answering the question of which side a country was on in the war can be surprisingly complicated and often depends on the period. This article provides an overview of the countries on the Allied side of the war, some of the key leaders, and the major events that shaped the political landscape of the war.

The Allies

Unlike the Axis, the Allies were not united by a treaty before the war. France and the United Kingdom—two of the Allies from World War I—declared war on Germany after the invasion of Poland in 1939. In spring and summer 1940, Germany invaded and defeated Belgium, the Netherlands, and France. Some members of their governments, including the French General Charles de Gaulle, fled abroad and declared themselves to be governments-in-exile, refusing to surrender and leading resistance movements against Nazi occupation from the outside.  

With the defeat of France in June 1940, virtually all of Western Europe fell under Nazi occupation. Britain was left to fight Germany with the support of its overseas empire including Canada, Australia, New Zealand, South Africa, and India. But the empire was far-flung across the globe, and Britain was reliant on shipping to provide needed supplies and men. German U-boats inflicted heavy losses on British ships and, in fall 1940, German bombers began attacking London in what became known as the Blitz.

In June 1941, Germany invaded the Soviet Union in violation of the Nazi–Soviet Pact, a 1939 non-aggression agreement that called for a period of extended peace between the countries and lay the groundwork for the division of Poland later that year. Despite the obvious ideological animosity between the anticommunist Hitler and the leader of the Soviet Union, Joseph Stalin, the leaders struck a deal that appeared to be in both of their short-term interests. Now this betrayal by Hitler brought Stalin into the war on the British side. The United States was still neutral but provided critical military equipment to both Britain and the Soviet Union through the Lend-Lease program.  

President Franklin D. Roosevelt conducted secret diplomacy with British Prime Minister Winston Churchill during this period and began to prepare the United States for possible entry into the war. However, American public opinion was still firmly against entering the conflict. In August 1941, Roosevelt and Churchill met secretly and issued a statement of postwar goals known as the Atlantic Charter, but this was not a formal alliance.

On December 7, 1941, Japan attacked the United States at Pearl Harbor. The United States responded by declaring war against Japan but not the other Axis powers. However, both Germany and Italy declared war against the United States on December 11, bringing America into conflict with all three Axis powers; the United States was now in a global war. Bulgaria, Hungary, and Romania declared war on America shortly after, but the United States did not declare war in return until June 1942.  

In December 1942, Churchill traveled to meet with Roosevelt personally in Washington, DC, and on January 1, 1942, they proclaimed the formation of the United Nations against the Axis powers. There were 26 original signers including the United States, the United Kingdom, the Soviet Union, and China; British dominions including Australia, Canada, India, and South Africa; numerous countries in South and Central Latin America; and various European governments-in-exile that had fled German occupation.  

Another 21 countries, including France, would join the United Nations before the end of the war. The leaders of the United States, the United Kingdom, and the USSR—Franklin D. Roosevelt, Winston Churchill, and Joseph Stalin—were often referred to as the “Big Three” as a reflection of their overall leadership.  

The term “Allies” can be applied to any country that fought as part of the United Nations during the war, including the United States, the United Kingdom, the Soviet Union, and China. Countries occupied by the Germans during the war including Poland, Norway, the Netherlands, France, and Belgium (and their respective empires) were aligned with the Allies when their governments-in-exile signed the United Nations Declaration or joined the alliance after liberation.

Outcomes from World War II

All the Axis powers were defeated in the course of World War II and left the conflict at different times. Mussolini was deposed following the invasion of Sicily in 1943, and Italy formally changed sides and joined the Allies later that year, though Rome would not be liberated until June 1944. Hitler initially propped up Mussolini as the head of a German puppet state in the north of Italy, but the Allied advance continued and in 1945 the former Italian dictator was captured and killed by partisans. Facing a Soviet onslaught in the East, Hungary, Romania, and Bulgaria each attempted to exit the war or join the Allies in 1944 as the Red Army approached and eventually occupied them.

Soviet forces entered Berlin in April 1945, prompting Hitler to commit suicide. Germany formally surrendered to the Allies on May 8, 1945, ending the war in Europe. Japan surrendered in August 1945 following the atomic bombings of Hiroshima and Nagasaki, marking the end of the Axis and the war itself. The postwar world would be shaped by the victorious Allies. In April 1945, the United Nations was formalized into the permanent international organization that bears the same name today.