Ally to

to unite formally, as by treaty, league, marriage, or the like (usually followed by with or to):
russia allied itself to france.
to -ssociate or connect by some mutual relationship, as resemblance or friendship.
to enter into an ; join; unite.
a person, group, or nation that is -ssociated 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 -ssociates or cooperates with another; supporter.
verb (əˈlaɪ) -lies, -lying, -lied usually foll by to or with
to unite or be united, esp formally, as by treaty, confederation, or marriage
(transitive; usually p-ssive) to connect or be related, as through being similar or compatible
noun (ˈælaɪ; əˈlaɪ) (pl) -lies
a country, person, or group allied with another
a plant, animal, substance, etc, closely related to another in characteristics or form
v.

late 13c., “to join in marriage,” from old french alier “combine, unite,” from a differentiated stem of aliier (from latin alligare “bind to;” see alloy). meaning “to form an alliance, join, -ssociate” is late 14c. related: allied; allying.
n.

late 14c., “relative, kinsman,” from ally (v.); mid-15c. in the sense of “one united with another by treaty or league.”