c. 1200, "a Roman legion," from Old French legion "squad, band, company, Roman legion," from Latin legionem (nominative legio) "Roman legion, body of soldiers, a levy of troops," from legere "to gather; to choose, pick out, select," from PIE root *leg- (1) "to collect, gather." Tucker writes that "The common sense is 'pick,'" but it is unclear whether the use here is "picking up or picking out." Roughly 3,000 to 6,000 men, under Marius usually with attached cavalry. "The legions were numbered in the order of their levy, but were often known by particular names" [Lewis].
The great power of the Roman legion was due to its rigid discipline and its tactical formation in battle, which was so open and flexible as to enable it to meet every emergency without surprise or derangement.
Generalized sense of "a large number of persons" (c. 1300) is due to translations of the allusive phrase in Mark v.9. Of modern military bodies from 1590s. American Legion, U.S. association of ex-servicemen, founded in 1919. Legion of Honor is French légion d'honneur, an order of distinction founded by Napoleon in 1802. Foreign Legion is French légion étrangère "body of foreign volunteers in a modern army," originally Polish, Belgian, etc. units in French army; they traditionally served in colonies or distant expeditions. Related: Legionary.