The War Resumes
Athens launched its aggressive campaign with an invasion of Sicily in 415 BC. The expedition began with Nicias, Lamachus, and Alcibiades as the chief commanders; however, the latter was recalled to Athens to face charges of impiety. He avoided the trial by fleeing to Sparta. Athens sent 200 ships on the expedition along with an estimated 650 cavalrymen. It was a reasonable force, but Syracuse had over 1,200 cavalrymen.
Aside from the setback of losing Alcibiades, the expedition began reasonably well. A victory at Catane was followed by what should have been a significant victory at the Battle of Syracuse in 415 BC. Instead of pressing their advantage, the Athenians abandoned their camp and returned to Catane. Perhaps the Athenians were concerned about potential harassment by the Syracusan cavalry; this could be the reason why its infantry failed to pursue its defeated enemy. Whatever the reason, it was a crucial error and one that perhaps turned the tide of the war; it certainly impacted the Sicilian invasion as the Syracusans were able to recover and persuade Sparta to declare war on Athens once again.
During the winter of 415-14, the Syracusans raided Athenian camps at Catane and forced their enemies to move to Naxos. The Corinthians sent aid to Syracuse and Sparta also agreed to send a small force. Unperturbed, Athens began its Siege of Syracuse in early 414 BC, and after a promising start, Lamachus was killed during the fighting. Nicias was left in sole charge, and he was apparently a poor commander (although he may have been ill) as the Spartan force under the leadership of Gylippus managed to sneak past the Athenians and join up with Syracuse.
With their combined power, they built a wall to prevent the Athenian attempt to blockade the city. In 413 BC, Athens won a naval and land battle but a second naval battle resulted in a shock defeat. An Athenian general called Demosthenes arrived on the scene with reinforcements but his ambitious attempt to attack the city ended in failure with heavy losses. Instead of retreating immediately, the Athenians followed superstition and insisted on waiting 27 days before leaving. This delay allowed the Syracusans to block their attempts to move by sea and land. The Athenian retreat was a complete disaster as its army was surrounded and forced to surrender. Nicias and Demosthenes were executed, and the survivors treated as slaves.