The battle in Sepia was fought, according to Herodotus, which is also the main source, in 494 BC. and resulted in Sparta gaining absolute sovereignty in the Peloponnese. Few details are known.
According to Herodotus who wrote more than 50 years after the battle, the Spartan king Cleomenes I moved towards Argos and arrived near Nafplio, at Sipia (Hispiia according to Herodotus). He camped there. The Argives came out to face the Spartans.
Cleomenes avoided the conflict and the two rivals remained facing each other. But the cunning Spartan ordered his men to be ready to attack when the signal to distribute rations was given.
The Argives, seeing the Spartans preparing to dine, quieted down and did the same.
But then Cleomenes gave the signal and the Spartans rushed out and began to slaughter their opponents. The Argives fled to the Sacred Forest of Argos to escape. But the Spartans set fire and their opponents met a tragic death.
About 50 Argives who took refuge in a sanctuary were misled and were massacred on their way out. Those who understood and refused to come out were also burned alive... According to Herodotus the Argives lost 6,000 of their men, 2/3 of the male population of the city! This was also one of the reasons why Argos did not participate in the war against the Persians a little later.
The traveler Pausanias tells the story of Argia Telesilla who saved the city after the disaster by arming women and slaves. Kleomenis, however, did not attack the city and was even summoned to an apology for this by the prefects.
It is said that he did not attack, considering it a humiliation to fight against women and slaves, and that even if he had won, he would have been laughed at in Greece. If he did not lose, the humiliation would be absolute. In any case the massacre at Sepia resulted in Argos losing Tiryns and Mycenae. However, the ferocity of the civil conflicts in ancient Greece was also demonstrated, which even exceeded the limits of sacrilege.