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 high regard for the diplomatic profession.
Sparta founded the first non-aggression pact in recorded history when it stopped seeking to conquer it's neighbors but sought defensive alliances with them instead. A series of bilateral treaties evolved into 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. (For more on the founding of the Peloponnesian League read The Olympic Charioteer.)
Sparta also sent an envoy to the Persian court late in the 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, one suspects) 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.
Sparta's diplomatic culure deserves much more attention and research. A comprehensive work on Spartan diplomacy would be a welcome addition to existing scholarly literature - or have I missed something? If anyone is aware of a good source on Spartan Diplomacy, please let me know.