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A hypaspist (Greek: Ὑπασπιστής "shield bearer" or "shield covered") is a squire, man at arms, or "shield carrier". In Homer, Deiphobos advances "ὑπασπίδια" or under cover of his shield. By the time of Herodotus (426 BC) the word had come to mean a high status soldier as is strongly suggested by Herodotus in one of the earliest known uses:
"Now the horse which Artybius rode was trained to fight with infantrymen by rearing up. Hearing this, Onesilus said to his hypaspist, a Carian of great renown in war and a valiant man..."
A similar usage occurs in Euripides play "Rhesus" and another in his "Phoenissae". Xenophon was deserted by his in a particularly sticky situation. A hypaspist would differ from a skeuophoros in most cases because the "shield bearer" is a free warrior and the "baggage carrier" was probably usually a slave. The word may have had Homeric and heroic connotations that led Phillip and Alexander of Macedon to use it for an elite military unit.
This unit, known as the Hypaspistai, or hypaspists were probably armed as hoplites rather than as phalangites or pikemen in Alexander the Great's Macedonian army. In battle they were probably armed with the Greek aspis shield, spolas or linothorax body-armor, Hoplite's helmet, greaves, dory spear and a xiphos or kopis sword (though their equipment was likely more ornate than main-line soldiers). In set piece battles the Macedonian Hypaspists were positioned on the flanks of the phalangite's phalanx, with the light infantry and cavalry, respectively, covering their flanks. Their job was guard the flanks of the large and unwieldy pike phalanx, an armored soldier with a 18-22 ft. pike (weapon). Pikemen are not particularly agile or able to turn quickly, so hypaspists would prevent attacks on the very vulnerable sides of the formation. Their role was vital to the success of Alexander's strategies because the Macedonian Phalanx is all but invulnerable from the front, and with five layers of iron spikes moving in unison at any one place, it was extremely formidable but also extremely inflexible. The pike phalanx was completely vital to the success of Alexander's strategies because he used it as the anvil in a hammer and anvil tactic, using his Companion cavalry as the hammer to smash the enemy against an anvil of thousands of inescapable iron spikes. As such an important yet vulnerable part of the Macedonian Army, it needed protection for its main vulnerability, the flanks. The protection/remedy for this vulnerability was the Hypaspists who were able to conduct maneuvers and use tactics, which, owing to their hoplite panoply of weapons and armor, would have been impossible (or at least much less effective) if performed by the Phalangites.
It is worth noting that all the references to a unit called "Hypaspists" are much later than the period of Alexander, and modern historians have to assume that later sources like Diodorus Siculus (1st century BC) and Arrian had access to earlier records.
Arrian's phrase tous kouphotatous te kai ama euoplotatous ) has frequently been rendered as 'lightest armed' although Brunt  concedes it is more properly translated as 'nimblest' or 'most agile'.
There has been a great deal of speculation by military historians since the late Hellenistic period about the elite units of Alexander's army. The hypaspists may have been raised from the whole kingdom rather than on a cantonal basis; if so, they were the King's Army rather than the army of the kingdom.
In the Hellenistic period the hypaspist apparently continued to exist, yet in different capacities and under different names. The name lived on in the Seleucid, Ptolemaic and Antigonid kingdoms, yet they were now seen as royal bodyguards and military administrators. Polybius mentions a hypaspist being sent by Philip V of Macedon, after his defeat at the Battle of Cynoscephalae in 197 BC, to Larisa to burn state papers.
The actual fighting unit of hypasists seems to have lived on in Macedonia as the corps of 'Peltasts', whose status, equipment and role seems to have been almost exactly the same as that of the hypaspist under Alexander. Originally consisting of 3,000 men by the Third Macedonian War they were 5,000, most likely to accommodate their elite formation, the Agema.
See also 
- Iliad Book 13, line 158
- Herodotus, Histories, 5.111
- Euripides Rhesus, line 2
- Xenophon, Anabasis 4.2.20
- Macedonian Warrior Alexander's elite infantryman,page 41,ISBN 978-1-84176-950-9,2006
- Diodorus Siculus Book 19.40
- Arrian's Anabasis Book 2 line 4 and following
- The Campaigns of Alexanderbook IV 28 viii
- The Campaigns of AlexanderAppendix XIX paragraph 9
- Polybius XVIII.33.1-7
- Polybius V.26.8
- Polybius V.25.1