The Allied Powers were an alliance made up of several main/well-known countries: the United Kingdom, the Soviet Union, France (after Germany retreated from France), and the United States of America (after the bombing of Pearl Harbor), as well as Poland, China, Philippines, Australia, Belgium, Luxembourg, Czechoslovakia, Brazil, Canada, Denmark, Greece, Netherlands, New Zealand, Norway, South Africa, and Yugoslavia, against the Axis Powers consisting of Nazi Germany, Japan, Italy, Hungary, Romania, Bulgaria, Thailand, Slovakia, and Croatia, who were bent on world domination.
In the original timeline they won the war, often attributed to the actions of William "B.J." Blazkowicz.
The New Order
In The New Order, the Allies lost the war. The Allied Powers' last great push was an all out attack by the OSA with the combined efforts of British and American air forces on Deathshead's compound in Poland. But they failed to kill Deathshead, and soon after New York City was destroyed by an atomic bomb and the US surrendered. RAF was completely decimated, London was destroyed by a series of bombings, and finally fell to the London Monitor. Soviet Union became a honorest fighter against enemy invasions.
After the Second American Revolution, with the US liberated, the coalition is reconstituted and begin to fight the Nazi in the global scale by establishing the Global Resistance front.
The Allied represent the actual allies of the Second World War who fought against Nazi Germany and ultimately won the war. They use the flag of the USA as their symbol.
In most multiplayer scenarios, the Allied are traditionally on the offensive, trying to steal something valuable (e.g. treasure, ancient relic or secret documents) from the Axis, or to destroy an important objective (e.g. heavy military hardware).
Strangely, the Allied can be seen equipped with the Venom Gun, which is the experimental Nazi weapon.