It was from the high ground of Epipolae that Syracuse was most exposed to attack. Nicias landed at Leon, a place upon the bay of Thapsus, at the distance of only six or seven stadia from Epipolae, took possession of Epipolae, and erected on the summit a fort called Labdalum. Then coming farther down the hill towards Syracuse, he built another fort of a circular form and of considerable size at a place called Syke. From the latter point he commenced his line of circumvallation, one wall extending southwards from Syke to the Great Harbour, and the other wall running northwards to the outer sea. The Athenians succeeded in completing the circumvallation towards the south, but in one of their many engagements with the Syracusans they lost the gallant Lamachus. At the same time, the Athenian fleet entered the Great Harbour, where it was henceforth permanently established. The northern wall was never completed, and through the passage thus left open the besieged continued to obtain provisions. Nicias, who, by the death of Lamachus, had become sole commander, seemed now on the point of succeeding. The Syracusans were so sensible of their inferiority in the field that they no longer ventured to show themselves outside the walls. They began to contemplate surrender, and even sent messages to Nicias to treat of the terms. This caused the Athenian commander to indulge in a false confidence of success, and consequent apathy; and the army having lost the active and energetic Lamachus, operations were no longer carried on with the requisite activity.

It was in this state of affairs that the Spartan commander, Gylippus, passed over into Italy with a little squadron of four ships, with the view merely of preserving the Greek cities in that country, supposing that Syracuse, and, with her, the other Greek cities in Sicily, were irretrievably lost. At Tarentum he learned to his great surprise and satisfaction that the Athenian wall of circumvallation at Syracuse had not yet been completed on the northern side. He now sailed through the straits of Messana, which were left completely unguarded, and arrived safely at Himera on the north coast of Sicily. Here he announced himself as the forerunner of larger succours, and began to levy an army which the magic of the Spartan name soon enabled him to effect; and in a few days he was in a condition to march towards Syracuse with about 3000 men. The Syracusans now dismissed all thoughts of surrender, and went out boldly to meet Gylippus, who marched into Syracuse over the heights of Epipolae, which the supineness of Nicias had left unguarded. Upon arriving in the city, Gylippus sent a message to the Athenians allowing them a five days' truce to collect their effects and evacuate the island. Nicias returned no answer to this insulting proposal; but the operations of Gylippus soon showed that the tide of affairs was really turned. His first exploit was to capture the Athenian fort at Labdalum, which made him master of Epipolae. He next commenced constructing a counter-wall to intersect the Athenian lines on the northern side. This turn of affairs induced those Sicilian cities which had hitherto hesitated to embrace the side of Syracuse. Gylippus was also reinforced by the arrival of thirty triremes from Corinth, Leucas, and Ambracia. Nicias now felt that the attempt to blockade Syracuse with his present force was hopeless. He therefore resolved to occupy the headland of Plemmyrium, the southernmost point of the entrance to the Great Harbour, which would be a convenient station for watching the enemy, as well as for facilitating the introduction of supplies. Here he accordingly erected three forts and formed a naval station. Some slight affairs occurred in which the balance of advantage was in favour of the Syracusans. By their change of station the Athenians were now a besieged rather than a besieging force. Their triremes were becoming leaky, and their soldiers and sailors were constantly deserting. Nicias himself had fallen into a bad state of health; and in this discouraging posture of affairs he wrote to Athens requesting to be recalled, and insisting strongly on the necessity of sending reinforcements.

The Athenians refused to recall Nicias, but they determined on sending a large reinforcement to Sicily, under the joint command of Demosthenes and Eurymedon. The news of these fresh and extensive preparations incited the Lacedaemonians to more vigorous action. The peace, if such it can be called, was now openly broken; and in the spring of 413 B.C. the Lacedaemonians, under King Agis, invaded Attica itself, and, following the advice of Alcibiades, established themselves permanently at Decelia, a place situated on the ridge of Mount Parnes about 14 miles north of Athens, and commanding the Athenian plain. The city was thus placed in a state of siege. Scarcity began to be felt within the walls; the revenues were falling off, whilst on the other hand expenses were increasing.

Meanwhile in Sicily the Syracusans had gained such confidence that they even ventured on a naval engagement with the Athenians. In the first battle the Athenians were victorious, but the second battle, which lasted two days, ended in their defeat. They were now obliged to haul up their ships in the innermost part of the Great Harbour, under the lines of their fortified camp. A still more serious disaster than the loss of the battle was the loss of their naval reputation. It was evident that the Athenians had ceased to be invincible on the sea; and the Syracusans no longer despaired of overcoming them on their own element.

Such was the state of affairs when, to the astonishment of the Syracusans, a fresh Athenian fleet of 75 triremes, under Demosthenes and Eurymedon, entered the Great Harbour with all the pomp and circumstance of war. It had on board a force of 5000 hoplites, of whom about a quarter were Athenians, and a great number of light-armed troops. The active and enterprising character of Demosthenes led him to adopt more vigorous measures than those which had been hitherto pursued. He saw at once that whilst Epipolae remained in the possession of the Syracusans there was no hope of taking their city, and he therefore directed all his efforts to the recapture of that position. But his attempts were unavailing. He was defeated not only in an open assault upon the Syracusan wall, but in a nocturnal attempt to carry it by surprise. These reverses were aggravated by the breaking out of sickness among the troops. Demosthenes now proposed to return home and assist in expelling the Lacedaemonians from Attica, instead of pursuing an enterprise which seemed to be hopeless. But Nicias, who feared to return to Athens with the stigma of failure, refused to give his consent to this step. Demosthenes then urged Nicias at least to sail immediately out of the Great Harbour, and take up their position either at Thapsus or Catana, where they could obtain abundant supplies of provisions, and would have an open sea for the manoeuvres of their fleet. But even to this proposal Nicias would not consent; and the army and navy remained in their former position. Soon afterwards, however, Gylippus received such large reinforcements, that Nicias found it necessary to adopt the advice of his colleague. Preparations were secretly made for their departure, the enemy appear to have had no suspicion of their intention and they were on the point of quitting their ill-fated quarters on the following morning, when on the very night before (27 Aug. 413 B.C.) an eclipse of the moon took place. The soothsayers who were consulted said that the army must wait thrice nine days, a full circle of the moon, before it could quit its present position; and the devout and superstitious Nicias forthwith resolved to abide by this decision.

Meanwhile the intention of the Athenians became known to the Syracusans, who determined to strike a blow before their enemy escaped. They accordingly attacked the Athenian station both by sea and land. On land the attack of Gylippus was repulsed; but at sea the Athenian fleet was completely defeated, and Eurymedon, who commanded the right division, was slain The spirits of the Symcusans rose with their victories; and though they would formerly have been content with the mere retreat of the Athenians, they now resolved on effecting their utter destruction. With this view they blocked up the entrance of the Great Harbour with a line of vessels moored across it. All hope seemed now to be cut off from the Athenians, unless they could succeed in forcing this line and thus effecting their escape. The Athenian fleet still numbered 110 triremes, which Nicias furnished with grappling-irons, in order to bring the enemy to close quarters, and then caused a large proportion of his land-force to embark.

Never perhaps was a battle fought under circumstances of such intense interest, or witnessed by so many spectators vitally concerned in the result. The basin of the Great Harbour, about 5 miles in circumference, in which nearly 200 ships, each with crews of more than 200 men, were about to engage, was lined with spectators. The Syracusan fleet was the first to leave the shore. A considerable portion was detached to guard the barrier at the mouth of the harbour. Hither the first and most impetuous attack of the Athenians was directed, who sought to break through the narrow opening which had been left for the passage of merchant vessels. Their onset was repulsed, and the battle then became general. The shouts of the combatants, and the crash of the iron heads of the vessels as they were driven together, resounded over the water, and were answered on shore by the cheers or wailings of the spectators as their friends were victorious or vanquished. For a long time the battle was maintained with heroic courage and dubious result. At length, as the Athenian vessels began to yield and make back towards the shore, a universal shriek of horror and despair arose from the Athenian army, whilst shouts of joy and victory were raised from the pursuing vessels, and were echoed back from the Syracusans on land. As the Athenian vessels neared the shore their crews leaped out, and made for the camp, whilst the boldest of the land army rushed forward to protect the ships from being seized by the enemy. The Athenians succeeded in saving only 60 ships, or about half their fleet. The Syracusan fleet, however, had been reduced to 50 ships; and on the same afternoon, Nicias and Demosthenes, as a last hope of escape, exhorted their men to make another attempt to break the enemy's line, and force their way out of the harbour. But the courage of the crews was so completely damped that they positively refused to re-embark.

The Athenian army still numbered 40,000 men; and as all chance of escape by sea was now hopeless, it was resolved to retreat by land to some friendly city, and there defend themselves against the attacks of the Syracusans. As the soldiers turned to quit that fatal encampment, the sense of their own woes was for a moment suspended by the sight of their unburied comrades, who seemed to reproach them with the neglect of a sacred duty; but still more by the wailings and entreaties of the wounded, who clung around their knees, and implored not to be abandoned to certain destruction. Amid this scene of universal woe and dejection, a fresh and unwonted spirit of energy and heroism seemed to be infused into Nicias. Though suffering under an incurable complaint, he was everywhere seen marshalling his troops and encouraging them by his exhortations. The march was directed towards the territory of the Sicels in the interior of the island. The army was formed into a hollow square with the baggage in the middle; Nicias leading the van, and Demosthenes bringing up the rear. The road ascended by a sort of ravine over a steep hill called the Acraean cliff on which the Syracusans had fortified themselves. After spending two days in vain attempts to force this position, Nicias and Demosthenes resolved during the night to strike off to the left towards the sea. But they were overtaken, surrounded by superior forces, and compelled to surrender at discretion. Out of the 40,000 who started from the camp only 10,000 at the utmost were left at the end of the sixth day's march, the rest had either deserted or been slain. The prisoners were sent to work in the stone-quarries of Achradina and Epipolae. Here they were crowded together without any shelter, and with scarcely provisions enough to sustain life. The numerous bodies of those who died were left to putrify where they had fallen, till at length the place became such an intolerable centre of stench and infection that, at the end of seventy days, the Syracusans, for their own comfort and safety, were obliged to remove the survivors, who were sold as slaves. Nicias and Demosthenes were condemned to death in spite of all the efforts of Gylippus and Hermocrates to save them.

Such was the end of two of the largest and best appointed armaments that had ever gone forth from Athens. Nicias, as we have seen, was from the first opposed to the expedition in which they were employed, as pregnant with the most dangerous consequences to Athens; and, though it must be admitted that in this respect his views were sound, it cannot at the same time be concealed that his own want of energy, and his incompetence as a general, were the chief causes of the failure of the undertaking. His mistakes involved the fall of Demosthenes, an officer of far greater resolution and ability than himself, and who, had his counsels been followed, would in all probability have conducted the enterprise to a safe termination, though there was no longer room to hope for success.