With the movement into the Hellenistic Age former methods of infantry organization and units began to change. The concept of the Hoplite started to quickly evolve into different categories of infantry. Other units that relied on firing and throwing missiles at the enemy would be used in coordination with the infantry. The once held notion of a pure Hoplite battle truly faded away and was done with during this period.
One of the largest factors in the change of infantry that armies would deploy was the fact that these kingdoms were now funding standing armies. State armies that had their gear and weaponry provided for them would be an economic burden if they were continued to be armed in the Hoplite fashion. To assist in countering this Hellenistic Kingdoms would maintain their standing force as an elite unit that would serve as the core of the army.
In examining the Seleucid Empire to help understand infantry in the time period their army’s elite were the Argyraspides (“Silver-shields”). This unit could be thought of as a Royal Guard; the king’s personal bodyguard consisting of 10,000 men that would be the core of the Seleucid Army. They continued the tradition of fighting in a phalanx formation serving as heavy infantry.
In addition to the Royal Guard there would also be other larger units of armed infantry that would fight in a phalanx formation. These units would not be as highly trained or equipped as the Silver-shields but they would nonetheless serve as an effective part of the main infantry body. The strength of the phalanx relied on maintain the formation and not allowing the enemy to flank or create any holes in the line. In modern terms these warriors could be thought of as reservists in the Seleucid military.
The adoption of light armed troops such as Peltasts would be major innovation in Hellenistic armies. Peltast troops were a blend of ranged javelin men and the heavier infantry of the phalanx. They were lightly armed troops that carried projectiles and could move ahead of the army quickly and harass their opponents with their missiles. Polybius also has Peltast fighting among the phalanx. Archers and slingers would also be a component of Hellenistic Armies. These units would be used to barrage their enemies with an array of arrows and stones attempting to thin enemy lines before the infantry engaged.
Other types of light troops that were in use were the Thorakitai. These units were valuable because they could provide a fast mobile force that did not require a fixed formation to fight. Mercenaries would also be an effective force hired by the Seleucids. While mercenary soldiers required large sums of money from the faction they would fight for they served as highly trained units. Because fighting in warfare was their careers they constantly trained and kept themselves well-armed.