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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.
Contemporary Examples

allying themselves with the Shiite militant group Hezbollah, some Lebanese Christians are standing their ground against ISIS.
Lebanese Christians Gun Up Against ISIS Susannah George November 9, 2014

On the other hand, it has restored drooping public support in allied countries for the idea of allying with Washington.
Obama as Hardheaded Liberal Michael Fullilove, Steven Casey August 26, 2011

News stories like this one at Vox expressed the consensus view that we were now allying with Assad.
Is Obama Done Playing Footsie With Assad? Michael Tomasky November 16, 2014

By allying himself with Iran hawks, Obama is getting the Iran-Syria dynamic exactly wrong.
Iran, Assad, and Obama’s Quagmire Peter Beinart September 8, 2013

Historical Examples

The promise of the Federation was that by allying themselves with men in party politics, women would soon earn the right to vote.
My Own Story Emmeline Pankhurst

I do not understand allyieg, unless it be ‘allying’ with the Irish.
Ireland under the Tudors, Volume I (of II) Richard Bagwell

The emperor wished to put an end to their dissensions by allying them to each other by marriages.
The Chinese Fairy Book Various

Mabel’s keen on allying the various branches of garment workers.
Comrade Yetta Albert Edwards

In some cities the nobility, allying itself with the proletariat, held the political power.
A Short History of Italy Henry Dwight Sedgwick

Your council has been over hasty in allying itself with these speculators.
The Waters of Edera Louise de la Rame, a.k.a. Ouida

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.”