An Ala (Latin for "wing", plural form: alae) was the term used during the mid- Roman Republic (338-88 BC) to denote a military formation composed of conscripts from the socii, Rome's Italian military allies. A normal consular army during this period consisted of 2 legions, composed of Roman citizens only, and 2 allied alae. Alae were somewhat larger than normal legions (ca. 5,400 v. ca. 4,500 men). From the time of the first Roman emperor, Augustus (ruled 30 BC - AD 14), the term ala was used in the professional imperial army to denote a much smaller (ca. 500), purely cavalry unit of the non-citizen auxilia corps, see Ala (Roman cavalry unit).
When, at a later date, the Roman armies were composed partly of Roman citizens and partly of Socii (allies from the rest of the Italian mainland), either Latini or Italici, it became the practice to marshal the Roman troops in the centre of the battle line and the Socii upon the wings. Armies of the middle republic would consist of two legions of Roman citizens and two legions of "ala", with the ala supplying thirty turmae of cavalry per legion, whereas the Roman provided only ten turmae. Hence ala and alarii denoted the contingent furnished by the allies, both horse and foot, and the two divisions were distinguished as dextera ala (right wing) and sinistra ala (left wing) (Livy, xxvii.2 , Livy, xxv.21 , Livy xxxi.21 ; Lips. de Milit. Rom. ii. dial. 7. We find in Livy x.40 , the expression cum cohortibus alariis ("with wing cohorts"), and in x. 43 , D. Brutum Scaevam legatum cum legione prima et decem cohortibus alariis equitatuque ire...jussit ("He ordered Decius Brutus Scaeva, legate, with the first legion and ten wing cohorts and the cavalry, to go and oppose said detachment...")..
This article is based on an article by William Ramsay, M.A., Professor of Humanity in the University of Glasgow on pp 73-74 of "A Dictionary of Greek and Roman Antiquities", John Murray, London, 1875, edited by William Smith, D.C.L., LL.D. This article is in the public domain. The information contained herein, as such, may therefore be outdated.