Clausewitz claimed that war was diplomacy by other means. This may explain why Sparta, popularly known as a militaristic society, was in fact a city with a long history of effective diplomacy and a high regard for the diplomatic profession.
Sparta’s diplomatic history started early. According to Herodotus, for example, Sparta sent an envoy to the Persian court mid-6th Century, long before the Persians had become interested in Greece. Allegedly, the Spartan envoy warned the Great King against enslaving Hellenes - which prompted the bewildered master of the Eastern world to ask who (in the hell) the Spartans were? After the diplomatic breech of murdering the Persian ambassadors sent to obtain earth and water in 491, the Spartans were concerned enough about diplomatic niceties to send to men to Persia as sacrifices to atone for the murdered ambassadors.
More significant, however, is the fact that Sparta founded the first non-aggression pact in recorded history when it stopped seeking to conquer its neighbors but sought defensive alliances with them instead. A series of bilateral treaties evolved into what became known as the Peloponnesian League. While initially this organization was an instrument of Spartan hegemony, which required Sparta's allies to follow her lead, in or about 500 BC the allies successfully asserted their power and effectively converted the League into an alliance in which every member - including Sparta - had an "equal" vote. Even if, as we know, some members of every alliance are always a little "more equal" than others, the principle of voting on major commitments of the Alliance was clearly established and largely respected.
Likewise, although often dismissed as simply “natural,” the forging of an alliance with Athens, Corinth, Aegina, (all hated rivals of one another!) and other lesser cities to oppose the Persian invasion of 480 was, in fact, a brilliant Spartan diplomatic achievement. I say Spartan, because the election of Sparta to lead on land and sea suggested that Athens would not have been able to hold this alliance together without Spartan influence. Arguably, it was Leonidas’ ability to put together a “coalition of the willing” to fight against Xerxes, more than his untimely death at Thermopylae, that was his greatest achievement.
Notably, some of Sparta’s best commanders were also excellent diplomats. Brasidas comes to mind. Brasidas had only limited troops, the bulk of which were helot volunteers without the training of Spartiates. His success depended not on force of arms, but his ability to win over allies and detach cities from the Delian League. Likewise Sparta’s success in Syracuse was certainly not a military success, no Spartan hoplites were in action at any time! It was, however, an enormous diplomatic success that severely weakened Athenian strength and morale.
The weaker Sparta became, the important it was for Sparta to forge alliances and out-wit rather than out-fight her enemies. It may be an indication of weakness, but it was nevertheless a diplomatic coup that Sparta was one of the first city-states in Greece to forge an alliance with a rising Rome, for example.
All in all, Sparta's diplomatic culture deserves much more attention and research. A comprehensive work on Spartan diplomacy from the Archaic to the Roman Periods would be a welcome addition to existing scholarly literature.