History of Iran: Parthian Army
The Greco-Persian wars and Alexander's victories proved that light-armed troops could not stop heavy, well-trained, and brilliantly led infantry of the type of hoplites or phalanx. These could only be encountered with heavily armed and highly professional cavalry causing disorder in the massed ranks and then attacking them on vulnerable points with bowshots capable of piercing armour and lances effective against shields. This lesson went home with the Parthians who in ousting the Seleucids from Iran had ample opportunity to experience the effect of heavily armed professional infantry led by Macedonian kings, and soon came to learn about the armament, tactics, and strategy of the Roman empire as well. So they formed their armies on sound bases, taking into consideration what was needed and what was available to them.