Then Alexander The Great crossed the Hellespont to launch his invasion of the Persian Empire in 334 D.C., he commanded an army of some forty thousand troops of mixed type. Herelied upon the Macedonian phalanx of heavy infantry, which his father had perfected, but he also employed both light and heavy cavalry, peltasts (light-armed infantry), skirmishers, archers and slingers, and other specialized troops. The soldier. themselves consisted of Macedonian citizens, allied Greek troops, and Greek mercenaries. By contrast, the army that the Spartan king Archidamus commanded in his invasion of Attica almost a century earlier, during the Peloponnesian War, consisted primarily of citizen-soldiers from Sparta and allied states. In short, Alexander's army was quite different in composition, organization, and diversity from the traditional armies of classical Greece. This factor surely counted for much in Alexander's amazing success in conquering Persia, although the generalship of the king himself was also obviously crucial.
Hamilton, Charles D.
"From Archidamus to Alexander: The Revolution in Greek Warfare,"
Naval War College Review: Vol. 48
, Article 7.
Available at: http://digital-commons.usnwc.edu/nwc-review/vol48/iss1/7