Pericles' plan to defeat Sparta seemed to have taken account of everything. With a fleet of 300 triremes; 13,000 hoplite infantrymen; 1,200 cavalry and 16,000 reserves; the Athenians believed themselves to be invincible. Such was their confidence that Pericles' main problem was preventing the assembly from rushing overconfidently into land battles, when his carefully constructed strategy relied on Athens weakening the enemy from the sea.

Then in 430, barely a year after the war began, Athens was struck by a disaster even Pericles could not have foreseen. The grain boats that fed the city brought with them an additional and deadly cargo - plague.

The plague spread through the overcrowded city of Athens like wildfire. Sufferers racked with fever and overcome with unquenchable thirst dived into the city's water cisterns, and there would die. Law and order broke down as the corpses of the dead piled up in the streets. An estimated 100,000 or more people were contained within Athens' great walls. By the time the first outbreak of the plague had run its course at least 20,000 of them were dead, including Pericles' two legitimate sons.