At the end of 416 BC, ambassadors from the city of Egesta in Sicily came to Athens, to ask for help against the Dorian city of Selinos, which was assisted from the city of Syracuse. General Nikias refused to accept such a dangerous and costly expedition, but the Athenian people led by Alkibiades took the decision to help them, since the city of Egesta proposed to undertake the expenses. At the beginning, it was voted to send 60 triremes under the leadership of Nikias, with Alkibiades and Lamachos, as generals. Nikias, who knew exactly how difficult was such an expedition, urged the Athenians to send a bigger fleet or cancel the project. Pressed by the Athenians, he proposed instead, a fleet consisting from 100 Athenian triremes, plus allied ships and a very large force of soldiers.
In the spring of 415 BC, the fleet was ready to depart, carrying about 1500 Athenians and 3500 allied hoplites, and about 1300 archers and slingers. The were ready to sail, but an unexpected event delayed it. One morning in May, the Athenians found out that the Herms, which stood at the entrance of temples and private houses, were broken. The event horrified the people and the name of Alkibiades was mentioned, but his enemies decided to bring charges in his absence.
In the morning of departure, the whole Athens came to the port of Piraeus to see the big armada and give them farewell. The fleet sailed at Corkyra first, where they were joined by their allies, bringing the total number of the ships to 134 triremes and two Rodian penteconters. The ships had on board 5000 hoplites, 480 bowmen, 700 Rodian slingers. The big armada was accompanied with at least 500 transport ships, which carried provisions.
On their arrival in Sicily, general Lamachos wanted an immediate attack against the city of Syracuse, but Alkibiades and Nikias refused. Alkibiades proposed a diplomatic campaign, which would bring them allies and thus the whole summer was wasted. In the meantime, Alkibiades was recalled to Athens to answer charges, particularly the profanation of the Eleusinian mysteries. During his return trip to Athens, Alkibiades escaped and later went to Sparta. Next year (414 BC) in the spring Nikias, who had exhausted all excuses for delay, prepared his army to attack Syracuse. After forcing the Syracusan army within the walls, he started constructing a double wall from sea to sea, preparing a blockade. The hero general Lamachos was killed during a battle, after trying to destroy the Syracusan fortifications.
Meanwhile, Alkibiades at Sparta did not waste time and advised them to renew the war with Athens, take hold of the strategic fort of Dekeleia and send a general to the city of Syracuse. The Spartans send general Gylippos with four ships. Though his force was small, he helped greatly Syracuse to win the war. He firstly captured the Athenian fort at Labdalum. This action made him master of the high ground Epipolae. He then constructed a counter wall to intersect the Athenian lines at the north side and so the Atenians from besiegers became besieged. This small participation of Sparta in the war was of the outmost importance.
Nikias in the hopeless position asked help from Athens, who send a big force consisting from 73 triremes and 5000 soldiers, under the able general Demosthenes. Upon his arrival Demosthenes and during the night tried to regain the high ground, but the operation failed. After this Demosthenes pressed Nikias to withdraw, but the general refused. Nikias was persuaded to withdraw, only when Syracuse was further reinforced, and his forces had deteriorated. But an eclipse of the moon, which occurred on August 27th, delayed the withdrawal and when the soothsayers were consulted, they told them to wait at least three days and others, for the next full moon. The delay gave time to the Syracusans, who upon learning that the Athenians were ready to retreat, lined up their fleet of 76 ships in the Great harbor, ready for battle. On September 3rd, 86 Athenian trieremes moved out to meet them. After hard battle the Athenians, who were at disadvantage, having no room for maneuvering their ships, lost the battle and their general Eurymedon was killed.
When the Syracusans blockaded the entrance of the bay with old ships (September 6th-8th), the Athenians tried once more to free themselves. On September 9th, general Nikias took the decision to attack the Syracusans once more and did everything possible to encourage the army visiting every trireme. The Athenians lined up their ships and attacked, but after a long and wavering battle, they were forced to the shore. General Demosthenes proposed to make another attempt to pass the barrier, but the soldiers refused to embark. The Athenians with many wounded people between them, abandoned their ships and tried to retreat into the interior. Their aim was to escape by land and reach the city of Katane. During their march the Athenians encountered the Syracusans many times, but at the end general Nikias surrendered the army to Gylippos, hoping to be treated fairly. General Nikias and Demosthenes were killed and the Athenian army were herded into the stone quarries of Syracuse. In terrible conditions, after eight months most of them died and the few that survived became slaves.
The disaster of the Sicilian expedition, it was a terrible blow, but Athens did not collapse. To make things worst, Dekeleia had been occupied by the Spartans, the silver mines at Laurio had been closed and nearly all the food was imported. In this situation, a peaceful revolution took place in Athens (411 BC), after one hundred years of democracy. The Council of Four hundred became the new administrative body, after the Athenian people were promised, that the change of the constitution was temporary, for the duration of the war. When the Lakedaemonian fleet defeated the Athenians near Eretria, in a small naval battle, the whole Euboea revolted and Athens lost the main supplier of food. After this incident, an assembly at the Pnyx, deposed the Four hundred and voted for a new government of five thousand leading citizens, who tried to make peace with Sparta, but without result. When the Athenian fleet and army based at Samos pressed for democracy, the oligarchs came in disagreement and within two years from the revolution, the democracy was restored.
In August of 411 BC, the Peloponnesian fleet commanded by Mindaros lost the naval battle at Kynossema. The Athenian fleet though smaller in force, in the straits of Sestos and Abydos, gained a complete victory.
In 410 BC, Alkibiades managed to capture the whole Peloponnesian fleet at Kyzikos. Mindaros was killed and the second in command Spartan sent a letter to the Ephors, in Laconic form: "Ships gone; Mindaros dead; men starving; no idea what to do."
Spartans were so discouraged, that they sent the Ephor Endius to Athens for a peace agreement but the Athenians, who were influenced by the demagogue Kleophon, rejected the offer. The victory at Kyzikos gave new hope to the Athenians, who restored their fleet and cut gold and copper coins. To give employment to the many skilled workers, the Athenians started building a new temple on the Acropolis, the Athena Polias, the so-called Erechtheion.
Spartans now appointed a new navarchos, the able man Lysander. When his turn of command expired, he was succeeded by Kallicratidas, who increased the number of ships of the Spartan fleet. There was a naval battle at the harbor of Mytelene with the Athenian fleet under Konon. The Athenians, who were outnumbered, lost the battle and thirty ships. Another forty ships were saved by bringing them ashore, near the walls of the town.
Kallicratidas then blockaded the island. When the news arrived at Athens they sent a fleet of one hundred and ten triremes and they were reinforced with another forty later. The number of ships of Kallicratidas were one hundred and twenty. At the small island of Arginusae, the Athenian fleet met the Spartan and after a hard struggle defeated them (406 BC). The Lakedaemonians lost seventy seven ships and the rest were retreated at Chios and Phocaea. Kallicratidas was thrown overboard, when his ship was hit by another and perished. The Athenians lost only twenty five ships.
Though it was illegal for an admiral to have a second term, Lysander, with the title of Epistoleus (bearer of letters), took the command of the Spartan fleet. He immediately obtained large sums of money from Kyros, king of Persia, to rebuild the fleet and made siege on Lampsakos.
The Athenians, who came to help, arrived too late to save the city and took post at Aegospotamoi (Goat's river) close to the city of Lampsacus. Lysander who systematically avoided a naval battle, since his ships were outnumbered, he managed to capture the Athenian fleet after treachery or negligence of the Athenian generals. All 4000 Athenian prisoners were put to death. This event substantially marked the end of Athens.