The posting below by Professor W. Sundermann’s discussion of the Arteshdar of the Sassanian army was orginally published in the Encyclopedia Iranica on December 15, 1986 (last updated by Iranica on August 15, 2011).
ARTĒŠTĀR (Middle Persian), a learned calque on and translation of the Avestan raθaēštā “warrior, war-hero” (Bartholomae, AirWb., col. 1506), which is a -tar formation, based on an original raθaēštā, literally: “in chariot standing, i.e. charioteer” (J. Kellens, Les noms racines de l’Avesta, Wiesbaden, 1974, pp. 231f.). This explanation of the Avestan forms is supported not only by the existence of the Sanskrit word ratheṣṭhā “charioteer” but also by the proper name Ratešda appearing in the Persepolis tablets, which must be its Old Persian equivalent (see M. Mayrhofer, Onomastica Persepolitana, Vienna, 1973, no. 8.1423). In the Middle Persian form, the long ā of the last syllable can be derived either from the accusative or from a secondary thematicization of the tar- stem (see Kellens, loc. cit.); and the initial a is attributed to analogy with artīk “fighting” by H. S. Nyberg (A Manual of Pahlavi II, Wiesbaden, 1974, p. 30, and Hilfsbuch des Pehlevi II, Uppsala, 1931, p. 2). The reading artēštār (with art- rather than arat-) is supported by New Persian artēšdār “soldier” (“miles” in J. A. Vullers, Lexicon Persico-Latinum I, repr. Graz, 1962, p. 76), though aratištār is found in Pāzand (Nyberg, Manual, loc. cit.), and ratēštār, which is nearer to the Avestan form, appears in the Pahlavi translation of the Avesta (B. N. Dhabhar, Pahlavi Yasna and Visperad, Bombay, 1949, p. 107; D. D. Kapadia, Glossary of Pahlavi Vendidad, Bombay, 1953, p. 268). If Nyberg’s theory is correct, the word artēštār must have been invented before the form of the word for “fighting” changed from artīk to ardīg, i.e. before the Sasanian period. The form nīsārīān in the Šāh-nāma (F. Wolff, Glossar zu Firdosis Schahname, Berlin, 1935, p. 832) was emended by E. Benveniste to artēštārī (“Les classes sociales dans la tradition avestique,” JA 221, 1932, p. 133).
Having entered the New Persian language in the form artēšdār (metrically artēšadār) or similar variants, the word was misunderstood as “artēš-having” (with -dār from dāštan “to have”).
The Farhangestān (Iranian Academy) established in Reżā Shah’s reign was misled by this misinterpretation of “artēš-dār” to see in the first component a Persian word arteš meaning “army.” This solecism was officially approved, and the Iranian army has been called the arteš ever since (Bozorg Alavi, Geschichte und Entwicklung der modernen persischen Literatur, [East] Berlin, 1967, p. 183; W. Eilers, Der alte Name des persischen Neujahrsfestes, Wiesbaden, 1953, p. 41 n. 1).
Archeological research has shown that use of chariots in the Iranian lands began in the 3rd millennium B.C. (R. Ghirshman, L’Iran et la migration des Indo-Aryens et des Iraniens, Leiden, 1977, pp. 15f.). The Avesta still speaks of charioteers, whom it calls raθaēštā, etc., cf. Yašt 10.112. In Yašt 10.11, however, the word must mean “warrior on horseback,” whence I. Gershevitch convincingly inferred that “the word had been generalized in the sense of “warrior”” (The Avestan Hymn to Mithra, Cambridge, 1958, p. 170). In Vidēvdāt 14.9, a list of 12 items of equipment required by a raθaēštā is given, with the implication that the word denoted any heavily armed rider and not any warrior. It is in this sense that raθaēštā is used in the Avesta as an aristocratic appellative of martial gods, such as Miθra, Sraoša, Ātar, and the Fravašis, Raθaēštā also denotes the second social class, the military nobility in the Avestan hierarchy, and it appears as an epithet of Zoroaster, who is already made to embody the virtues of all classes as the téleios ánthrōpos (K. Barr) “the perfect man,” being the first priest, warrior, and cultivator (Yt. 13.88-89; see also below).
All these aspects of the Avestan raθaēštā are echoed in the descriptions of the artēštārs in the Pahlavi literature. Their clothing and equipment are portrayed in the Ardā Wirāz-nāmag (ed. M. Haug and E. W. West, chap. 14.7-10). The Avestan comparisons of particular deities with artēštārs are further developed, especially in the case of the Mēnōg ī Āsmān (Spirit of Heaven) who protects the world of light and whose solid armor, consisting of the heavenly spheres, is likened to the armor of an earthly warrior (Bundahišn, pp. 18.15-19.2). Similarly the frawahrs of the just who throng the heaven are warriors “who ride dashing steeds and wield lances” (Bundahišn, pp. 60.15-61.3; Wizīdagīhā ī Zādspram, ed. B. T. Anklesaria, chap. 3.3). The first to wear the attire of an artēštār was the valiant Wāy ī weh (Bundahišn, p. 235.8-13). Artēštār (and artēštārīh) continued to denote the nobility in Zoroastrian terminology (e.g. Škand Gumānīg Wizār, ed. J. de Menasce, 1.22, 27-28). Duties, virtues, and failings of artēštārs are discussed in several Pahlavi texts (e.g. Dēnkard, book 3, ed. Madan, p. 325, ed. M. Dresden, [248-49]; Mēnōg ī Xrad, ed. E. W. West, 31.2.9, and 59.3.81). A chapter of the Sasanian Avesta, Artēštāristān “warrior code” (West), is set forth at length in book 8 of the Dēnkard (ed. Madan, pp. 729.12-732.12, ed. M. Dresden,  (last line)- line 13; cf. Wizīdagīhā ī Zādspram 28.4). In the Sasanian period the revered fire-temple of Ādur Gušnasp (Taḵt-e Solaymān) in Azerbaijan was considered to hold the special fire of the artēštār class; as in the Avesta, Zoroaster was depicted as the perfect priest, warrior, and cultivator (M. Boyce, Zoroastrianism, p. 278; Dēnkard, book 7, ed. M. Molé, La légende de Zoroastre selon les textes pehlevis, Paris, 1967, 1.41; Wizīdagīhā ī Zādspram 31.5), and his son Xwar-čihr was regarded as the “first” warrior and the eschatological army commander of Pēšōtan (Bundahišn, p. 235.8-13). It is also stated, however, that before Zoroaster, Jamšēd, the primeval king, possessed the good qualities of all the classes (Dēnkard, book 7, ed. Molé, 1.20); and this statement accords with the New Persian-Arabic literary tradition, which makes Jamšēd the founder of the class system.
The artēštār concept alone, however, played no practical part in Sasanian society, where only the office of the artēštārān sālār (q.v.) had any real importance.