What is the opposite of allies?

The opposite of allies (on your side) would be enemies.

In World War I, the "Allies" were the Triple Entente and later the US and Italy, and were opposed by the Central Powers (Germany, Austria-Hungary, and the Ottoman Empire).

In World War II, the major "Allied powers" were Britain, France, the USSR, and later the US, opposed by the Axis Powers (Germany, Italy, and Japan).