"One Ally" is kind of ... difficult. "It's me! I'm allied with myself! I shall refer to myself as The Ally!"
There were several nations in an alliance, so they were, for reasons one hopes are self-evident, called "the Allies".
the invasion of Ethiopia
No. The United States fought against Hitler to defeat Germany and the Nazis during WWII. England was an ally of the United States during the war.
Ally
The ANZACS fought with the UK during both wars, so any ally of Britain was an ally of Australia and New Zealand. (i.e United States, France, Canada etc)
Ally is singular. Allies means more than one Ally.
Allies.
Allies.
Allies.
of Ally
the US is afghanistans ally
Ares was Aphrodite's ally.
Romania has not true allies.