Infantry in the Persian army, and more precisely in the army of the Persian state under the Achaemenid dynasty (from approx. 550 BC to approx. 330 BC), played an important role, performing various functions - from scouting to fighting in the open field in progress battles. Most often it was placed in the center of the Persian army's combat formation. Its exact number is only an estimate - the numbers given by the author of "The Acts" of Herodotus are rather perceived as unrealistic. It is assumed, however, that the number of Persian infantry during Xerxes' expedition to Greece (480-479 BC) could be from 100,000 to 200,000. people. However, these are still estimates. Undoubtedly, the elite of the Persian infantry was the Immortals unit, always composed of 10,000, recruited from the native Persians and Medes. This unit is certainly certified in sources during the reign of Cambyses (533-520 BC), and perhaps it was formed by his father - Cyrus the Great (559-530 BC). The soldiers of this formation were very well trained, had high morale and were armed with spears, bows and probably short swords. On the other hand, they had marginal protective equipment, which brought a deplorable effect during the battles at Marathon (490 BC) or Thermopylae (480 BC). It was not until the Greco-Persian wars that the soldiers of this formation probably received lamellar armor.
The Spartan army was considered the best in the Greek world of the classical period, especially in the 6th-5th centuries BC. She wore this title deservedly. Its number at the beginning of the Greco-Persian wars, and therefore in the beginning of the 5th century BC, is estimated at 9,000-10,000 soldiers, led by one of the Spartan kings. A full-fledged citizen of the Spartan polis, or Spartiat, was getting ready to fight and broadly understood war from the age of 7, when his military training (Greek: agoge) began. It lasted until the age of 21 Spartiata and included constant physical exercise, getting the future warrior used to the hardships of combat, hunger and cold, but also taught camaraderie and strict discipline. As a result of agoge, Spartan soldiers had a very high morale, unattainable in other Greek armies, literally iron discipline, but they were also de facto professional soldiers who knew their craft very well. In the period of the 6th-5th centuries BC, they fought in the formation of the phalanx. Each of them was a hoplite equipped with a hoplon, a bronze helmet (very often of the Corinthian type), armor (e.g. the so-called athlete's armor) and greaves, but also a spear and a sword. The Spartan army showed great courage and loyalty to the commander (King Leonidas) above all in the Battle of Thermopylae (480 BC), while its incredibly high combat value - in the Battle of Plataea in 479 BC.