Meaning

noun a friendly nation

noun an associate who provides cooperation or assistance

verb become an ally or associate, as by a treaty or marriage

originFrom Middle English allien, alien (“to form an alliance, associate, join; to become an ally; to introduce (someone) as an ally; to marry; to become related (to someone); to attack, engage in combat; to combine; (cooking) to combine ingredients, especially to bind them together”) [and other forms], from Anglo-Norman alier, allier, Middle French alier, allier [and other forms], and Old French alier…

Etymology from Wiktionary, CC BY-SA 3.0