Should Allied Be Capitalized?

Longman dictionary -- the following word is capitalised - Allies. However, my friend says it is not a proper noun and so strictly should not be capitalised.

Is allied a proper noun?

Proper noun

The countries allied against the Axis Powers during World War II. The major signatories included the Soviet Union, Britain, France, United States, Canada, and China. The countries allied against the Central Powers during World War I, including especially the United Kingdom, the Russian Empire, and France.

Do you capitalize Allied and Axis Powers?

Also, notice that the names of the sides of a war, such as the Allies or the Axis Powers, are capitalized as well. Names of historical periods and documents are also capitalized.