v. To place in a friendly association, as by treaty: Italy allied itself with Germany during World War II. v. To unite or connect in a personal relationship, as in friendship or marriage.verb-intransitive. To enter into an alliance: Several tribes allied to fend off the invaders. n. One that is allied with another, especially by treaty: entered the war as an ally of France. n. One in helpful association with another: legislators who are allies on most issues. See Synonyms at partner.n. The nations allied against the Central Powers of Europe during World War I. They were Russia, France, Great Britain, and later many others, including the United States.n. The nations, primarily Great Britain, France, the Soviet Union, and the United States, allied against the Axis during World War II.