Battle of Gaugamela main Persian infantry was poorly trained and equipped in comparison to Alexander's pezhetairoi and hoplites. The only respectable infantry Darius had were his 2,000 Greek hoplites and his personal bodyguard, the 10,000 Immortals. The Greek mercenaries fought in a phalanx, armed not with a heavy shield but with spears no longer than three metres, while the spears of the Immortals were two metres long. Among the other Persian troops, the most heavily armed were the Armenians, who were armed the Greek way and probably fought as a phalanx. The rest of Darius' contingents were much more lightly armed; the main
Thracian warfare though this must not have been universal among them. The Triballi had adopted Celtic equipment. Another weapon, the sica was called "Thracian sword" (Ancient Greek,"Θρακικον ξίφος") though it did not originate from there, despite its popular usage (it was considered their national weapon). The sword's utmost origin was the Hallstatt culture and the Thracians may have adopted or inherited it. Greece affected Thracian warfare early on with the xiphos and other swords, Greek type greaves, breastplates, a variety of helmets and other equipment. During the Hellenistic period more Greek armaments were adopted. Seuthes had adopted a Greek tactic for a