In general, allies are people or groups that have joined an alliance and are working together to achieve some common purpose. In general English usage, those who share a common goal and whose work toward that goal is complementary may be viewed as allies for various purposes even when no explicit agreement has been worked out between them. Similarly, when the term is used in the context of war or armed struggle, a formal military alliance is not required for being perceived as an ally — co-belligerence, to fight alongside someone, is enough. According to this general usage, allies become allies not when concluding an alliance treaty but when struck by war.
In the context of diversity politics, an ally has been defined as "a person of one social identity group who stands up in support of members of another group; typically a member of dominant group standing beside member(s) of a group being discriminated against or treated unjustly; e.g., a male arguing for equal pay for women." (This definition is adapted from one developed by the Arizona State University Intergroup Relations Center).
When spelt with a capital A, Allies usually denotes the countries that fought together against the Central Powers in World War I and against the Axis powers in World War II. The term is generally used in the generic sense of "all who opposed the enemy". In addition, it is usually used in a strict dichotomy of them vs. us, reflecting wartime propaganda, with no account taken of nuances of countries that were occupied as neutrals, changed sides or participated in concurrent wars.
In previous major European wars, e.g., those against the declarers of war Louis XIV of France, Louis XV of France, and Napoleon, the term coalition was used because these were not considered total wars, and the sovereign nations could enter and leave belligerency with diplomatic agreements with the enemy.
World War I
France, Russia and Britain joined the war as the Triple Entente.
US President Woodrow Wilson and his administration were keen not to define USA as an ally. The U.S. entered the war as an "associated power" rather than as an ally of France and Britain, and maintained that distance through the war and the Paris Peace Conference, 1919.
(Norway is at times referred to as "The Neutral Ally". While theoretically a neutral country, British pressure and anti-German sentiment in the population enabled the government to highly favour Britain in matters concerning the large Norwegian shipping fleet and vast fish supplies.)
World War II
After Nazi-Germany in March 1939 had occupied the remains of Czechoslovakia, the British ambassador was recalled from Berlin and Neville Chamberlain declared that if Hitler attacked Poland, considered next in turn for an assault by the Third Reich, then the UK and France would give Poland "all support in their power", a promise soon also given to Greece and the later Axis member Romania after Italy's conquest of Albania on April 7, 1939.
A formal military alliance was concluded between the UK, France and Poland on April 6th, 1939, whereafter also the Soviet Union initiated alliance negotiations, although unsuccessfully. The Soviet Union would instead agree with Nazi-Germany in the Molotov-Ribbentrop Pact of August 1939. In the latter part of the war the Allies were often referred to as "the United Nations" from the 1942 Declaration by the United Nations. The name was given to what was initially known as the United Nations Organization (later the United Nations) when it was formed following the war.
The dates given below are for entry into the war.
These countries were major pre-war powers, and in the post-war years were the permanent members of the UN Security Council. However, the inclusion of France here may be regarded as contentious, since it surrendered to Nazi Germany on June 22, 1940, and the forces of Free France were numerically much smaller than those of some "other allies" listed below. (Vichy France became a de facto Axis member, following the destruction of the French Fleet at Mers-el-Kebir on July 3, 1940.)
The War in Europe began when Poland was invaded on 1 September, 1939.
Several countries of the British Commonwealth declared war on Germany separately, either on the same day, or soon after Britain:
The vast majority of Canadian and South African forces were involved in the European Theatre. Australia was initially involved in the European campaign, but from 1941 most Australian forces were gradually re-deployed to defend their homeland, which was in the Pacific Theatre. However, most New Zealand forces remained in Europe.
Most countries occupied by the Axis powers continued the fight with resistance movements and/or governments in exile:
Belgium (invaded by Germany, May 10, 1940)
Czechoslovakia (seized by Germany, September 29, 1938 as part of the Munich Agreement)
Denmark (invaded by Germany, April 9, 1940)
Free France (from Charles De Gaulle's Appeal of June 18, 1940)
Greece (invaded by Italy, October 28, 1940)
Netherlands (invaded by Germany, May 10, 1940)
Luxembourg (invaded by Germany, May 10, 1940)
Norway (invaded by Germany, April 9, 1940)
Poland (invaded by Germany and Russia in September 1939)
Yugoslavia (invaded by Germany, Italy, Hungary, Romania and Bulgaria, April 6, 1941)
Poland, technically, had a government exiled in 1939, but the Polish contribution to World War II included hundreds of thousand members of her armed forces.
British, Dutch and French colonies fought alongside their mother countries, and many continued their contribution also when the mother countries were occupied.
From July 1944 a Brazilian Expeditionary Force (with at most 25,000 men) participated in the Allied invasion of Italy.