At the Battle of Sepeia (; c. 494 BC), the Spartan forces of Cleomenes I defeated the Argives, fully establishing Spartan dominance in the Peloponnese. The Battle of Sepeia is infamous for having the highest number of casualties within a battle during the classical Greek period. Herodotus provides context of the political, military and economic landscape of the Peloponnese before, during and after the Battle of Sepeia. During the Battle of the 300 Champions (546 BC) Sparta gained control over this region. Nearly two generations had since passed when Argos reclaimed Thyrea.
The events that occurred after this held significant religious undertones. Once the Oracle of Delphi had foretold the success of Sparta against Argos, Cleomenes led his armies to the Erasinos River on the border of the Argolid. proclaiming: “I admire the God of the River for refusing to betray his countrymen. But the Argives will not get away that easily” and left.
In Thyrea, Cleomenes made another sacrifice to the sea to access the river via a different route. The transportation route Cleomenes arranged was a challenging maritime task to fulfil, involving the transportation of the Spartan forces across the díolkos at Corinth.
thumb|266x266px|Bust of Herodotus in Palazzo Massimo (Rome)
Modern historians have attempted to calculate a date for the battle based on other passages from Herodotus. When the Argives visited the Oracle of Delphi, two predictions were told; the demise of Argos and that of the Milesians,
Strategic and Tactical Deliberations
The strategic and tactical deliberations of the Battle of Sepeia begin with Spartan’s march to Argos. Initially, Cleomenes led the Spartans to the Erasinos River, on the border of the Argolid, When Cleomenes was questioned for his breach of the truce; the King argued the truce was made for seven days and didn’t include nights. However, similar to Plutarch, Cicero describes a situation in which a general plunders the fields at night, because the truce had been made for days and not nights. and deceived the Argives to their death. Cleomenes announced that the hoplites who’d been ransomed for the standard fee were free to leave. Consequently, Cleomenes set fire to the grove, slaughtering any survivors who attempted to escape.
The Battle of Sepeia and the sacrilege that ensued resulted in six thousand Argive casualties.
Due to the nature and ambiguity pertaining to historical sources and administration during the time period, an accurate estimation of the Argive population is improbable. If the assumption was made that we could assume a similar Argive population during the Battle of Sepeia, the losses of six thousand would have compromised a large majority of young and middle-aged Argive men.