To be formally associated, as by treaty
To unite or be united in a relationship
One nation associated with another in a common cause
One who is united in a relationship with another
See syn. study at
"I am not your ally," he repeated.
You're my only true ally of any influence with the Council.
The odd little man was the loyal ally A'Ran considered him!
Germany, an ally of Austria-Hungary, was obligated by treaty to defend it.
Our new friend may be an ally to Sirian.