The wikipedia entry for "Hoplite" states that:
"The short sword was a secondary weapon, used if or when their spears were broken or lost, or if the phalanx broke rank. The xiphos usually had a blade around 60 centimetres (24 in) long; however, those used by the Spartans were often only 30–45 centimetres long. This very short xiphos would be very advantageous in the press that occurred when two lines of hoplites met, capable of being thrust through gaps in the shieldwall into an enemy's unprotected groin or throat, while there was no room to swing a longer sword. Such a small weapon would be particularly useful after many hoplites had started to abandon body armour during the Peloponnesian War"
Is this true, and if so, what prompted the abandonment of armour?
Hello, so many historians have pointed to a downward trend in armour from the archaic all the way to the Hellenistic era amongst Hoplites / Spear infantry. This trend is very hard to pin down and is difficult to be very certain about, but there are some pretty major changes in archaeological finds and iconographic evidence of Hoplites. Further complicated by the fact that its a bit of a debate exactly how much armour an 'average' Hoplite wore. However, the biggest and most obvious changes are in helmet design, from the Corinthian to several other designs that had more open faces and sometimes holes for the ears etc. The second most obvious change is a movement from bronze to a composite breastplate, which was lighter. Now, since equipping oneself for war was a personal endeavour in the ancient world (for the most part), one could choose your own designs and these are trends, not always firm in their dating or consistency. As for why? Well this is likely tied to an increase in usage of light troops (however, again this is a debate, let me know if you want to know more). During the peloponesian war Hoplites get defeated or badly mauled by light troops a few times Thuc. 4.33 for one example. Lightening their armour could have increasing manoeuvrability and been an attempt to deal with a more complex and mobile battlefield (again this issue is difficult). Thats a shortish answer as im not sure how much of an indepth answer you want.
Snodgrass. 1967. Arms and armour of the Greeks. London : Thames & Hudson Snodgrass, A. 1964. Early Greek Armour and Weapons: From the End of the Bronze Age to 600 B.C. Edinburgh: Edinburgh University Press Rawlings, L 2000. Alternative Agonies: Hoplite Martial and Combat Experiences Beyond the Phalanx. In: Wees, V.H.(ed). War and Violence in Ancient Greece, Aberystwyth: Classical Press of Wales, 233-259. Hanson, V.D. 1989. The Western way of war. London: Hodder and Stoughton Kagan, G and Viggiano, P. 2013. Men of Bronze hoplite Warfare in Ancient. Princeton : Princeton University Press 2013 (if you want more book or sources just let me know)
You might find some additional interesting stuff in the answer I wrote to this question here, with more exploration of the historiography and some different suggestions as to why the gradual lightening of hoplite equipment happened.