Big Four (World War II) in the context of "World War II by country"

The history of the Soviet Union (USSR) (1922–1991) began with the ideals of the Russian Bolshevik Revolution and ended in dissolution amidst economic collapse and political disintegration. Established in 1922 following the Russian Civil War, the Soviet Union quickly became a one-party state under the Communist Party. Its early years under Lenin were marked by the implementation of socialist policies and the New Economic Policy (NEP), which allowed for market-oriented reforms.

The rise of Joseph Stalin in the late 1920s ushered in an era of intense centralization and totalitarianism. Stalin's rule was characterized by the forced collectivization of agriculture, rapid industrialization, and the Great Purge, which eliminated perceived enemies of the state. The Soviet Union, one of the Big Four Allied powers alongside the United States, the United Kingdom, and China, played a crucial role in the Allied victory in World War II, but at a tremendous human cost, with millions of Soviet citizens perishing in the conflict.

Almost every country in the world participated in World War II. Most were neutral at the beginning, but relatively few nations remained neutral to the end. World War II pitted two alliances of nations against each other, the Allies and the Axis powers. It is estimated that 74 million people died, with estimates ranging from 40 million to 90 million deaths (including all genocide casualties). The main Axis powers were Nazi Germany, the Empire of Japan, and the Kingdom of Italy; while the United Kingdom, the United States, the Soviet Union and China were the "Big Four" Allied powers.

The countries which were either involved in or affected by World War II are listed alphabetically, along with a description of the role which they played in the conflict.

The history of the United Nations has its origins in World War II, beginning with the Declaration of St James's Palace. Taking up the Wilsonian mantle in 1944–1945, US president Franklin D. Roosevelt pushed as his highest postwar priority the establishment of the United Nations to replace the defunct League of Nations. Roosevelt planned that it would be controlled by the United States, Soviet Union, United Kingdom and China. He expected this Big Four would resolve all major world problems at the powerful Security Council. Since then its aims and activities have expanded to make it the archetypal international body in the early 21st century.