 According to this usage, allies become so not when concluding an alliance treaty but when struck by war.When spelled with a capital "A", "Allies" usually denotes the countries who fought together against the Central Powers in World War I (the Allies of World War I), or those who fought against the Axis Powers in World War II (the Allies of World War II).The term has also been used by the United States Army to describe the countries that gave assistance to South Vietnam during the Vietnam War.