By the time that Plato's Republic was published (according to Scott Buchanan in the Introduction to The Portable Plato), Plato seems to have concurred with his rival, Isocrates, that the Greek city-states should formally agree (in the event of war between or among whatever states) upon certain "civilized" rules for the conduct of warfare. A prelude to the fixing of such an agreement seems to be Plato's intent in his discussion of the conduct of the Guardians (Book V) in the event of internecine discord. In the Notes to The Republic of Plato, Francis MacDonald Cornford notes that, although Plato "expresses no humanitarian sympathy extending beyond the borders of Hellas," Plato is one of the earliest writers to stand for a rule of international law between independent states. Plato, Isocrates, and other thinkers were fully aware of the narrow escape from subjugation the Greeks had been granted at the close of the Persian war in 479 b.c. — a war waged against Persia by an alliance of Hellenic city-states including Athens and Sparta, who not many years later would be at war against each other. These thinkers had been nurtured on stories of the Persians' barbarity during the conduct of that war, barbarity practiced upon heroic Greek warriors such as the Spartan Leonidas.
According to the ancient Greek historian Herodotus in The History of Herodotus, Leonidas was in command of the now famous 300 Spartans who were sent, in advance of the main Spartan body of troops, to engage the Persian horde in order to arrest and defeat its intended invasion of Greece. The Spartan troops of only 300 men at arms were augmented by troops sent by several other city-states who seemed determined to engage the Persian forces, but Leonidas had picked up and accompanied the delegation of troops from Thebes, because the Thebans had already hinted that they might desert the Greek alliance and unite themselves with the Persians. In order to shore up the flagging hopes of their allies, therefore, the Spartan advance guard made camp in a narrow mountain pass at a place now made famous by the battle fought there — Thermopylae (the Hot Gates).
