The battle of Pylos (425 BC) was the first part of a two-part battle most famous the surrender of a force of Spartan hoplites trapped on the island of Sphacteria (Great Peloponnesian War). The occupation of Pylos wasn't the result of an official Athenian policy. In 425 BC the city if Messina on Sicily had revolted against Athens, and in response the Athenians decided to send a fleet to Sicily. Demosthenes was not one of the generals appointed to command this fleet, but he was allowed to accompany the fleet and suggest operations that they could carry out while sailing around the Peloponnese.
Demosthenes already had a plan in mind when he joined the fleet. He wanted to occupy the rocky headland of Pylos in the south-west of the Peloponnese, fortify it and use it as a base to raid Spartan territory. The headland was forty-five miles to the west of Sparta in territory that had once belonged to the Messenia, a city that had been Sparta's great local rival two centuries earlier before being conquered and destroyed. Demosthenes believed that the Messenians would provide a garrison for the stronghold, which was also close to a good natural harbour and well provisioned with building materials.
Demosthenes first suggested this plan while the fleet was at sea, but the two generals rejected it. Bad weather then forced the fleet to take shelter at Pylos. Demosthenes repeated his suggestion, and was again rebuffed. According to Thucydides the plan was saved by the Athenian soldiers, who without prompting from Demosthenes began to fortify the headland, largely because they were bored during the enforced delay. The close proximity of the city of Sparta and its famed warriors may have had more to do with their sudden enthusiasm for building work, but once the fortifications had been completed the generals relented, and gave Demosthenes five ships and their crews to act as his garrison.
The occupation of Pylos took place during the annual Peloponnesian invasion of Attica. When the news reached the Spartan king Agis, he decided to end the invasion early (after only fifteen days) and return home to deal with the new threat. His army was also running short of provisions and suffering from unseasonably wet weather, so the landing at Pylos wasn't the only reason for the short campaign in Attica. After returning from Attica Agis sent a force of Spartans from the city and surrounding communities to Pylos, while his army followed behind. A Spartan fleet based further north at Corcyra was also summoned, and managed to slip past an Athenian squadron based on Zakynthos. Demosthenes was aware that he was about to be besieged, and in turn summoned the fleet from Zakynthos.
The Spartans decided on both a short-term and a long-term plan. The short-term plan was simply to attack the Athenian fortifications, in the belief that they would easily fall. The long-term plan took advantage of one weakness of the Athenian position. The rocky headland at Pylos was at one end of Pylos Bay. The island of Sphacteria runs north-south across the mouth of the bay, creating a good natural harbour, but also limited access to the narrow channels at either end of the island (the northern channel had room for two ships to pass, the southern was wider, with room for eight or nine). The Spartans decided to moor their ships in rows in these two channels, with the prows facing outwards. Part of their army would blockade the Athenians on the mainland and part would be posted on the island.
Demosthenes was faced with an unexpected situation. He should have expected to have been outnumbered on land, but he couldn't have expected to be outnumbered at sea. He decided to leave most of his troops in the land fortifications, but selected sixty hoplites to defend what he believed was the most likely landing point. Demosthenes himself joined the beach party.
The Spartans attacked exactly where Demosthenes had expected. The naval force consisted of 43 ships led by the Spartan Thrasymelidas son of Cratesciles, but the beach was too small for all of them to attack at once. Only a few ships could attack at any one time, negating the Spartan's numerical advantage, although it did allow them to rotate their troops. The future Spartan general Brasidas came to the fore during this stage of the fighting, but despite his best efforts the Spartans were unable to establish a foothold on the beach. The attacks on the land walls also failed, and after two days of attacks the Spartans decided to pause and construct siege engines.
The tables were soon to be turned. While the Spartans were waiting for their siege engines the Athenian fleet was approaching. By the time it arrived it contained fifty ships, more than enough to defeat the Spartan fleet. To make things worst the Spartans had failed to carry out the naval part of their plan, so their fleet was beached inside the Bay of Pylos. Nothing happened on the day that the Athenians arrived, but on the following day they sailed into the bay and inflicted a heavy defeat on the Spartans. The Spartans were able to prevent them from towing some empty ships off the beach, but that was the limit of their success.
The Athenian naval victory meant that the last detachment of Spartans on Sphacteria was now trapped. This force of 420 Spartan hoplites, under the command of Epitades son of Molobrus, soon found itself under attack, and much to the shock of the Greek world chose to surrender rather than fight to the death (battle of Sphacteria).