In 416 BC, Alcibiades became a champion of sending a fleet to aid the Sicilian city of Segesta which had been an Athenian ally since 420 BC. Segesta had lost the war to Selinus, which was allied to Corinth, a component of the Sparta led Peloponnesian league.
Nicias took a stand against Alcibiades. He argued that the war would be an ill-advised adventure. There was a heated debate between him and Alcibiades. Nicias attacked Alcibiades, calling him an inexperienced, self-aggrandizing man who was leading Athens into a futile war. In his response, Alcibiades recounted his own virtues and talked about the successes that he had achieved for Athens in the past. Being a better talker than Nicias, Alcibiades managed to convince the Athenian Assembly to vote for war. Having failed to stop Athens from going to war, Nicias recommended that the departure of the expedition be delayed but Alcibiades insisted that this was the right time to launch the war.
In 415 BC, Athens sent out its largest overseas expedition since their 455 BC expedition to Egypt (which had ended in a disaster for the Athenian side). The Athenian thinking was that they needed to make gains in Sicily since that area had the potential to become a decisive theatre of operations for the Peloponnesian War, which had been raging since 431 BC.
The original plan was to let a board of three generals—Alcibiades, Lamachus, and Nicias—lead a naval expedition in which too many Athenian lives would not be put to risk. But in the second meeting of the Assembly, the Athenians vastly increased the scope of their expedition. They voted for a fleet of 135 triremes, several cargo ships, and a large army. It was clear that the guise of aiding Segesta, the Athenians were aiming to capture Syracuse.
On the night when the expedition was about to depart, there was an incident in Athens that was interpreted as an ill omen. Someone mutilated the stone markers representing Hermes, the guardian male figures that stood around the city for good luck. The mutilation could have been the work of the Athenian faction which was against the war and wanted to delay the expedition. The rumor was spread that the associates of Alcibiades were responsible. He was not charged for the act of vandalism and the expedition was allowed to leave the next day. In 415 BC, Syracuse was divided. The Syracusans did not believe that Athens would send a massive expedition to Sicily and they had not made any preparation to defend themselves. A united Athenian military leadership might have captured Syracuse. But the Athenians were not united.
Since the Assembly had failed to define the aims of the war, a clash between the three generals was inevitable. The three generals came up with three different strategies. Nicias proposed a minor battle, followed by return to Athens. Alcibiades proposed that they should try to win over allies in Sicily and then attack Selinus and Syracuse. Lamachus proposed that they should directly attack Syracuse. The Athenian fleet was divided into three sections, one for each general.
Contrary to what the Athenians had expected, most cities of Sicily did not welcome the Athenian expedition. Only two cities, Leontini and Segesta, received the Athenians. Another problem was that Segesta, which had earlier promised that it would pay for the expedition, declared that it did not possess the funds. On learning this, Nicias recommended that they should make a show of force in the area and then proceed to Athens. But Alcibiades advised that the right course of action would be to encourage revolts against Syracuse, and then attack Syracuse and Selinus. Lamachus continued to insist that Syracuse must be attacked immediately.
Alcibiades managed to conquer Catania, but before he could conquer more cities in Sicily, a ship arrived from Athens and he was informed that he was under arrest for the crimes of destruction of the Hermai and profaning the Eleusinian Mysteries. Alcibiades promised to return to Athens to stand trial. But he gave the prosecutors a slip and took a ship to the Peloponnese, where he sought refuge in Sparta. His flight to Sparta was taken by the Athenians as a proof of his guilt, and an Athenian court sentenced him to death.
The number of Athenian generals in Sicily now came down to two: Nicias, who was against the war and had a reputation for inaction, and Lamachus, who was not a popular figure though he had a credible military record. A significant part of the summer had been wasted and in the fall of 415 BC, Nicias finally agreed to attack Syracuse. They faced no opposition while entering the harbor and managed to land their army at river Anapus. There was a hoplite battle between the Athenian and the Syracusan side. The Syracusans lost around 260 men, and the Athenians lost 50. After this encounter, the Athenians moved to Catania to spend the winter.
In the spring of next year, the Athenians were back in Syracuse. This time they landed on the Epipolae, the cliff above Syracuse. In the fighting that ensued, 600 Syracusan soldiers were killed. The Athenians began the construction of a series of walls known as “the circle,” with which they hoped to blockade Syracuse from the rest of the island. The Syracusans responded by building their own series of counter-walls to connect their various forts. The Athenians managed to destroy one counter-wall but the Syracusans built another wall, this time with a ditch which blocked the Athenians from extending their own wall to the sea. A group of 300 Athenian soldiers attacked the new Syracusan counter-wall and captured it, but eventually they were beaten back by the Syracusans.
Lamachus was killed during the Syracusan counteroffensive, leaving Nicias as the sole commander of the expedition. The Syracusans destroyed about 1000 feet of the Athenian wall, but they could not destroy the circle, which was defended by Nicias. The Athenians managed to extend their wall to the sea and tightened their blockade of Syracuse.
The Syracusans appealed to Sparta for help. The Spartans nominated General Gylippus to command their expedition for Syracuse. Nicias’s failure to finish construction of the wall provided Gylippus with an opportunity to land the Spartan forces at Himera and march them overland to the Syracusan city. Gylippus put his men to work in helping the Syracusans in building the counter-walls. In the first engagement between the Athenians and the Spartans, the Athenians drove the Spartans back, but in the second attempt the Spartans defeated the Athenians. The work for building the counter-walls went on. Once the Syracusan counter-walls were completed, the Athenian walls became useless. The Syracusan and Spartan side was bolstered with the arrival of a fleet from Corinth.
The Athenian expedition was in trouble. Nicias should have called off the expedition and returned to Athens. But he sent a letter to the Athenian Assembly claiming that he was too sick to command the expedition and he needed assistance. He was hoping that the Assembly would call off the expedition To his surprise they responded by sending another expedition to Syracuse, commanded by the general Demosthenes (not the philosopher). In 413 BC, Demosthenes arrived with 73 ships and 5,000 hoplites. He was appalled to see that Nicias had allowed the situation in Syracuse to deteriorate to such an extent.
Demosthenes realized that there were two options left: either capture Epipolae or retreat to Catana. In a risky night-operation (which was opposed by Nicias), Demosthenes attacked the Syracusan counter-walls on Epipolae. His troops breached the wall but they were defeated by the Spartans on the other side. In the darkness many Athenians became disoriented and, according to Plutarch, around 2000 of them fell to their death from the cliff. After this debacle, Demosthenes advised Nicias that they should return to Athens and defend their homeland. But Nicias did not accept this sage advice. He was worried that if he retreated at this stage, the Athenians would execute him for cowardice and incompetence. He insisted that they should continue to fight.
In a series of naval engagements, the Syracusans destroyed several Athenian ships. In the battles being fought on land, the Spartans led by Gylippus inflicted heavy casualties on the Athenians. The Athenian situation became perilous. They decided to burn the rest of their ships and march towards Catana through the river Anapus. On the way, they were continuously attacked by the Syracusans. The ferocity of the Syracusan attacks forced Demosthenes and Nicias to change the direction of their retreat to the south. But the Syracusans continued to come after the retreating Athenians. In the next two days hundreds of Athenians were slaughtered.
At the river Erineus, Demosthenes and Nicias became separated. After a short battle, Demosthenes and his 6000 soldiers surrendered to the Syracusans. The troops with Nicias fled towards the river Assinarus with the Syracusan soldiers in hot pursuit. There was a breakdown of order on the Athenian side—in their rush to find drinking water, many soldiers were trampled to death. Tempers flared and fights broke out between the Athenians. They ended up killing several of their own soldiers. The Syracusan soldiers who caught up with the fleeing Athenians were in no mood to show mercy. They cut down thousands of Athenian soldiers.
Against the orders of the Spartan general Gylippus, both Demosthenes and Nicias were executed by the Syracusans. A few Athenian soldiers who had survived the massacre were kept for days in a horrible prison, where they slowly died of disease, thirst, and hunger. Most accounts suggest that Athenians had lost 10,000 hoplites and 30,000 experienced oarsmen. Considering the fact that the Athenian population was just 150,000, this was a massive loss. The Athenians had also lost most of their triremes. When the news of the military disaster reached Athens, the population revolted. In 411 BC, the Athenian democracy was overthrown and power went to an oligarchy.