The "Archidamian War"

Sparta and its allies, with the exception of Corinth, were almost exclusively land-based powers, able to summon large land armies which were very nearly unbeatable (thanks to the legendary Spartan forces). The Athenian Empire, although based in the peninsula of Attica, spread out across the islands of the Aegean Sea; Athens drew its immense wealth from tribute paid from these islands. Athens maintained its empire through naval power. Thus, the two powers were relatively unable to fight decisive battles.

The Spartan strategy during the first war, known as the Archidamian War (431-421 BC) after Sparta's king Archidamus II, was to invade the land surrounding Athens. While this invasion deprived Athenians of the productive land around their city, Athens itself was able to maintain access to the sea, and did not suffer much. Many of the citizens of Attica abandoned their farms and moved inside the long walls, which connected Athens to its port of Piraeus. At the end of the first year of the war, Pericles gave his famousFuneral Oration (431 BC).

The Spartans also occupied Attica for periods of only three weeks at a time; in the tradition of earlierhoplite 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 periods of time. The longest Spartan invasion, in 430 BC, lasted just forty 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 most dominant in Greece, went on the offensive, winning a victory 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 course of the first war turned in Athens's favour. The post off Pylos struck Sparta where it was weakest: its dependence on the helots. Sparta was dependent on a class of slaves, known as helots, to tend the fields while its citizens trained to become soldiers. The helots made the Spartan system possible, but now the post off Pylos began attracting helot runaways. In addition, the fear of a general revolt of helots emboldened by the nearby Athenian presence drove the Spartans to action. Demosthenes, however, outmanoeuvred the Spartans in the Battle of Pylos in 425 BC and trapped a group of Spartan soldiers on Sphacteria as he waited for them to surrender. Weeks later, though, Demosthenes proved unable to finish off the Spartans. After boasting that he could put an end to the affair in the Assembly, the inexperienced Cleon won a great victory at the Battle of Sphacteria. The Athenians captured between 300 and 400 Spartan hoplites. The hostages gave the Athenians a bargaining chip.

After these battles, the Spartan general Brasidas raised an army of allies and helots and marched the length of Greece to the Athenian colony of Amphipolis in Thrace, which controlled several nearby silver mines; their product supplied much of the Athenian war fund. Thucydides was dispatched with a force which arrived too late to stop Brasidas capturing Amphipolis; Thucydides was exiled for this, and, as a result, had the conversations with both sides of the war which inspired him to record its history. Both Brasidas and Cleon were killed in Athenian efforts to retake Amphipolis (see Battle of Amphipolis). The Spartans and Athenians agreed to exchange the hostages for the towns captured by Brasidas, and signed a truce.