In 421 b.c.e., with the establishment of the Peace of Nikias, Athens and Sparta managed to set a provisory truce on the Peloponnesian War (431–404 b.c.e.). However, a period of mutual suspicions and instability followed, which created new conflicts for both poleis and their allies.

In the winter of 415 b.c.e. the Sicilian city of Segesta decided to ask for Athenian support against their neighbor Selinus, which was helped by Syracusan forces. According to Thucydides (in his History of the Peloponnesian War), Athens agreed to organize and send 60 ships to the island under the joint command of Alcibiades, Nikias, and Lamakhos. Full power was granted to the three generals in order to assist Segesta.

Strong debates were held concerning the opportunity and the need of the expedition, and Nikias expressed his view against the inconvenience of sending troops far from Athens with an imperialist goal. However, the people supported Alcibiades in favor of the expedition.

Shortly before the departure of the fleet a shocking event interpreted as a bad omen took place: It is recorded that in one night almost all of the statues of Hermes in the city (known as the Hermai) were mutilated. This violent destruction of the busts was seen as a clear act of conspiracy against the state. Alcibiades was suspected in the episode, and claims for responsibility were presented against him. But he had a popular image and was not immediately charged.

The huge fleet, composed by Athenian and allied triremes, constituted the biggest military expedition ever conducted in classical Greece. They left Athens in June 415 b.c.e. and, after joining other forces in Corcyra, reached Sicily.

As soon as it was clear that Segesta had no money to support the military deployment, the generals clashed: Whereas Nikias proposed a return to Athens, Lamakhos wanted an immediate attack in Syracuse, and Alcibiades suggested some initial negotiations with the enemy. Informed about his condemnation in Athens, Alcibiades decided to escape to the Peloponnesian islands, where he contacted the Spartans.

However, the other two generals followed his plan and decided to put off the main attack. They settled their fleet in Catana, where Syracusan troops under the command of Hermokrates prepared for combat. A first encounter between cavalry forces occurred, and the Syracusans had to flee the battle camp.

During the winter both parties discussed alliances with different cities in Italy, reinforced their military capacity, and built defensive walls. Called in to help by the Syracusans, a Spartan contingent under Gylippos arrived in the city and successfully held some skirmishes with the Athenians, turning the tide. The Athenians received reinforcements from Demosthenes and Eurymedon. Nonetheless, this support was not enough to overcome the local horsemen.

A mistake by Nikias, who decided to postpone the Athenian homecoming, sealed their defeat: Syracusan and Spartan vessels took advantage of the situation, attacked the rival ships in the harbor, pushed them into the shore, and started a blockage. Athenians decided to leave camp, guided by Nikias and Demosthenes. But they were obliged to split troops in two, and after final encounters with Syracusans, both generals were forced to surrender (413 b.c.e.).

Many Athenians were massacred, a few escaped and asked for refuge in Catana, and the remaining were taken as prisoners under harsh conditions. The effects of this catastrophic expedition put the polis at stake.

Even if Athens was able to go on with the war against Sparta for another nine years, the truth is that the Sicilian disaster entailed an absolute loss of power and clearly represented the beginning of its final military and political decadence.