The battle was fought between the two most powerful city-states on the Peleponnesos, Sparta and Argos, for dominance over the peninsula.
The two sides had clashed before, most heavily at the battle of Hysia in 669 BCE,
where Argos had won because the army of that city had started to develop a larger, heavier shield and used a tight formation.
Their style of fighting was the forerunner of the hoplite style.
By the time the battle of 300 champions was fought, hoplites had fully developed and were the mainstay of both sides.
Spartans and Argives had skirmished frequently, but then decided to settle the dispute in a pitched battle.
Rather than risk their entire army, each side sent 300 hoplites, so were numerically exactly equally matched.
The rest of the armies even marched home, so they could not intervene or help the 600 in any way, not even care for the injured.
According to Herodotus, the two armies battled mercilessly until nightfall, until only three men remained, two Archives and one Spartan.
But the latter was heavily wounded and the Archives thought he was dead.
Thinking that they had won the day, they raced back to their city to proclaim victory.
But the Spartan, Orthryades, packed himself up.
He was the last man standing on the battlefield, so claimed victory too.
It is said that he was ashamed to be the only one of his unit to survive and thus committed suicide.
This view is doubted; he may well have succumbed to his wounds shortly after his moment of triumph.
What is important is that he did not die by the hands of the Archives.
Both sides now claimed victory.
They failed to settle this new dispute by negotiation, so another battle was fought.
This time the whole armies took part.
Sparta won and took control of Thyreatis, but the rivalry between the two city-states was not over.
They fought a major battle again in 505 BCE, which Sparta won, to gain a definitive upper hand on the Peleponnesos.
In 420 BC, during a lull in the Peloponnesian War, Argos challenged Sparta to a rematch of the Battle of the 300 Champions, but it declined.
War Matrix - Battle of 300 champions
Iron Age 1100 BCE - 550 BCE, Battles and sieges