to unite formally, as by treaty, league, marriage, or the like (usually fol. by with or to )

Russia allied itself to France.

to associate or connect by some mutual relationship, as resemblance or friendship.

to enter into an alliance; join; unite.

a person, group, or nation that is associated with another or others for some common cause or purpose

Canada and the United States were allies in World War II.

Biology . a plant, animal, or other organism bearing an evolutionary relationship to another, often as a member of the same family

The squash is an ally of the watermelon.

a person who associates or cooperates with another; supporter.

noun

antagonist , enemy

noun

accessory , accomplice , associate , coadjutor , collaborator , colleague , confederate , co-worker , friend , helper , partner , coalitionist , leaguer , affiliate , cohort , copartner , fellow , abettor , assistant , auxiliary , backer , comrade , connect , join , relative , supporter , unite

verb

align , confederate , federate , league , affiliate , bind , combine , conjoin , connect , join , link , relate