Pylos/Sphacteria

In the winter of 426/425, Demosthenes had noticed that a place in the southwestern Peloponnese called Pylos (not to be confused with the palace of the Homeric hero Nestor) could easily be fortified, and that the Athenians could use it as a base for further raids in the region. Moreover, this part of the Peloponnese, Messenia, was hostile against the Spartans, who had subdued the inhabitants, had made helots of them, and terrorized them. The Athenian garrison at Pylos offered them an opportunity to escape. This would greatly damage the Spartan economy. Demosthenes' proposal was imaginative, and the statesman Cleon, a "hawk", was able to see to its financing. 

This was the most spectacular Athenian victory during the war. Immediately, the Spartans offered a truce, because they were unwilling to sacrifice their men. They proposed a peace treaty and good will for the future, but Cleon immediately brushed it aside: there was no guarantee that the Spartans would not change their mind later. If they wanted peace, they needed to offer something better, including some sort of certainty for future peace. So, the war was resumed, but it was a different war: it had been shown that Sparta would stop fighting when its own people were imperiled, and would betray its allies by concluding a peace treaty. Sparta had lost the moral high ground.

Still, many Athenians thought that Cleon had made a mistake, and he was more or less forced to create an even bigger victory. And so he did. He went to Pylos, spoke to Demosthenes, and attacked the Spartans on the island, who in the end surrendered. This was another blow for the Spartans that sincerely handicapped them, because they could no longer attack Athens - the hostages would be executed.

Although our most important source, the History of the Peloponnesian War by Thucydides, a personal enemy of Cleon, presents this as a lucky victory, it was in fact a carefully planned operation, as we can read in the World History by Diodorus of Sicily (text). "Pylos" not only marked the end of the myth of Spartan invincibility, but also showed that it was not willing to risk Spartan lives. On the other hand, Sparta was now forced to embark upon other strategies, and this opened the road to the resourceful Brasidas, who was to become the most dangerous enemy of Athens.