Allies spelled with a capital "A", usually denotes the countries who fought together against the Central Powers in World War I (see Triple Entente or Allies of World War I), or those who fought against the Axis Powers in World War II (see Allies of World War II).
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.