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
Ancient Greek warfare undermanned. However, the lightly armored Persian infantry proved no match for the heavily armored hoplites, and the Persian wings were quickly routed. The Greek wings then turned against the elite troops in the Persian centre, which had held the Greek centre until then. Marathon demonstrated to the Greeks the lethal potential of the hoplite, and firmly demonstrated that the Persians were not, after all, invincible. The revenge of the Persians was postponed 10 years by internal conflicts in the Persian Empire, until Darius's son Xerxes returned to Greece in 480 BC with a staggeringly large army (modern estimates suggest between