Fans of “300” may find it hard to think of Leonidas as a diplomat. In the Hollywood cartoon, Leonidas is portrayed as the brutal antithesis of a diplomat: he personally throws a Persian ambassador down a well. But there is no more historical evidence that Leonidas committed this crime than that Xerxes was a monster. The historical record, foggy and imprecise as it is, suggests that far from being a tactless brut, Leonidas was probably a very savvy diplomat.
The evidence for Leonidas’ diplomatic talent is indirect rather than explicit. It is evident in what he did, rather than what is said about him. Quite simply: During his brief reign, Leonidas managed to forge a coalition of Greek states willing to oppose the Persian invasion and to convince this loose coalition of independent and proud city-states to agree to a unified command and a joint strategy. The significance of such an achievement can be measured by the fact that ten years earlier Athens didn’t place her army under the unified command of even a single Athenian; no less than ten generals shared command of the Athenian army at Marathon. Equally notable, while Leonidas’ brother Cleomenes alienated Lacedaemon’s Peloponnesian allies to the point of provoking revolt, Leonidas won over new Allies such as Mycenae and Tiryns.
As for the incident with the Persian ambassadors, Herodotus tells us that the Spartans shared the guilt for the murder of the ambassadors. According to Herodotus, the entire city was threatened by ill-omens and the Spartan Assembly met repeatedly in order to find volunteers from among the citizens willing to appease the Gods by dying in atonement for the murdered Persian ambassadors. If, as when Cleomenes’ burned the Sacred Wood near Argos, the crime had been committed by either of the Spartan kings, the Spartans would have expected/demanded that the king bear responsibility. Whoever killed the Persian ambassadors, the entire Spartiate population felt collectively guilty about it – something that suggests the Persian emissaries had not been the victims of a spontaneous act of violence but rather condemned by the Spartan Assembly. (Something which in turn suggests that Spartan Assemblies could be quite rowdy affairs, but that is a subject for another day….)