Attica for periods of only three weeks at a time; in the tradition of earlier hoplite warfare, the soldiers were expected to go home to participate in the harvest. Moreover, Spartan slaves, known as helots, needed to be kept under control, and could not be left unsupervised for long. The longest Spartan invasion, in 430 BC, lasted just 40 days. The Athenian strategy was initially guided by the strategos, or general, Pericles, who advised the Athenians to avoid open battle with the far more numerous and better trained Spartan hoplites, relying instead on the fleet. The Athenian fleet, the dominant Greek naval force, went on the offensive, winning at Naupactus. In 430 BC, an outbreak of a plague hit Athens. The plague ravaged the densely packed city, and in the long run, was a significant cause of its final defeat. The plague wiped out over 30,000 citizens, sailors and soldiers, including Pericles and his sons. Roughly one-third to two-thirds of the Athenian population died. Athenian manpower was correspondingly drastically reduced and even foreign mercenaries refused to hire themselves out to a city riddled with plague. The fear of plague was so widespread that the Spartan invasion of Attica was abandoned, their troops being unwilling to risk contact with the diseased enemy. After the death of Pericles, the Athenians turned somewhat against his conservative, defensive strategy and to the more aggressive strategy of bringing the war to Sparta and its allies. Rising to particular importance in Athenian democracy at this time was Cleon, a leader of the hawkish elements of the Athenian democracy. Led militarily by a clever new general Demosthenes (not to be confused with the later Athenian orator Demosthenes), the Athenians managed some successes as they continued their naval raids on the Peloponnese. Athens stretched their military activities into Boeotia and Aetolia, quelled the Mytilenean revolt and began fortifying posts around the Peloponnese. One of these posts was near Pylos on a tiny island called Sphacteria, where the first war turned in Athens's favor. The post off Pylos exploited Sparta's dependence on the helots, slaves who worked the fields while its citizens trained to be soldiers. The Pylos post began attracting helot runaways. In addition, the fear of a revolt of helots emboldened by the nearby Athenians drove the Spartans to attack the post. Demosthenes outmaneuvered the Spartans in the Battle of Pylos in 425 BC and trapped a