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 tribal wars also kept Thrace from becoming a major regional power due to the lack of a central government. At the onset of the Peloponnesian War, the Thracian tribes were united under the rule of Sitalces, king of the Odrysae. It was allied with Athens during the conflict. However, the kingdom was again split into different parts after the death of king Seuthes. It was during this time when Philip of Macedon conquered a large swath of Thrace, absorbing the territory and its tribes into Macedon. The "Odrysian kingdom" (Ancient Greek, "Βασιλεία Όδρυσων") was a union of Thracian tribes that