The attack had been six years in the making, and still, the Persians were not fully prepared.
War between the Persians and Macedonians had become inevitable when Persia had supported the Perinthians’ resistance against Macedonian aggression in 340. When the Macedonian king Philip II had secured his rear in the battle of Chaeronea (338), he wanted to launch a campaign east of the Hellespont. As fate would have it, the Achaemenid king Artaxerxes III Ochus died (of natural causes) in the same summer, leaving the Persian Empire without strong successor. His son Artaxerxes IV Arses had to cope with revolts in Babylonia (Nidin-Bel), Egypt (Chababash), and Armenia (Artašata). It seemed that everything was ready for the attack – except that Philip was assassinated in 336. At that moment, the Macedonian vanguard, commanded by Parmenion and Attalus, was already in Asia.
Philip was succeeded by his son Alexander, who secured his position in 335 and joined his vanguard in 334. At that moment, Artaxerxes IV had been murdered and Artašata, calling himself Darius III Codomannus, was in charge. He quickly suppressed the revolts in Babylonia and may have been occupied in Egypt when the Macedonian main force finally arrived in Asia.
The attack had been six years in the making, and still, the Persians were not fully prepared. Employing local levies and Greek mercenaries, they had been able to drive back the Macedonian vanguard, but facing the main force was something else; the Macedonians were to be superior in numbers and equipment.
The local satraps and Memnon chose a defensive position at the Granicus. This was a clever move. Alexander wanted to conquer the Greek cities in Ionia (western Turkey) in the south; the Persians now forced him to move to the west, unless he wanted his lines of communication to be cut off. Moreover, they would have the benefit of Dascylium as their supply base. The Macedonian were to fight under conditions dictated by the Persians.
There are two accounts of the battle. Arrian and Plutarch say that Alexander attacked directly across the river (Anabasis, 1.13; Life of Alexander, 16). This maneuver, however, is impossible, because the banks are too steep for horses to climb – and little has changed since Antiquity. The other story is told by the historian Diodorus of Sicily (World History, 17.19.3), who says that the Macedonians crossed the Granicus during the night, and attacked the Persians on the plain beyond the river before dawn. This was a good stratagem: the Persians were accustomed to sacrifice to the rising sun before they armed themselves, and would be unprepared.
Arrian’s description of the Persian battle array also suggests that Alexander had made sure he could cross the river safely first: the Persians put their cavalry in front of their infantry, which is madness if they expected the attack across the river, but makes sense if the cavalry had to protect the infantry against the superior numbers of the Macedonian phalanx. The Persian tactic was to defeat the Macedonian cavalry first, which would enable them to surround the Macedonian phalanx, and compensate their superior numbers by attacking them in the rear.
The Persians must have known that something was wrong when they heard the sound of the approaching phalanx. The cavalry rushed forward and there was a brief cavalry fight in which Clitus saved Alexander’s life. The outcome, however, was never in doubt: already during the cavalry fight, the infantry lines clashed, putting an end to Persian hopes to be able to defeat the Macedonian cavalry and attack the Macedonian phalanx from two sides. The Persian army was destroyed; the last to resist were the Greek mercenaries. They were probably massacred.
The consequence of the battle was that the road to the south was open to Alexander; Sardes and Ephesus were taken without fight. Dascylium and Gordium, other Persian strongholds, were unprotected too. Alexander was not to encounter resistance until the Persian navy appeared at Miletus.