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 associate or connect by some mutual relationship, as resemblance or friendship.
to enter into an ; 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.
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 passive) 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, associate” 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.”