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According to Thucydides, the cause of the war was the "fear of the growth of the power of Athens" throughout the middle of the 5th century BC . After a coalition of Greek states thwarted an attempted invasion of the Greek peninsula by the Persian empire, several of those states formed the Delian league in 478 BC in order to create and fund a standing navy which could be used against the Persians in areas under their control. Athens, the largest member of the league and the major Greek naval power, took the leadership of the league and controlled its treasury. Over the following decades, Athens was able to convert the Delian league into an Athenian empire. This increase in Athenian military power allowed it to challenge the Lacedaemonians (commonly known as the Spartans), who, as leaders of the Peloponnesian League, had long been the sole major military power in Greece.
The immediate cause of the war comprised several specific actions of Athens that affected Sparta's allies, notably Corinth. The Athenian navy intervened in a dispute between Corinth and Corcyra, preventing Corinth from invading Corcyra at the Battle of Sybota, and placed PotidaeaThe Battle of Potidaea was, with the Battle of Sybota, one of the catalysts for the Peloponnesian War. It was fought near Potidaea in 432 BC between Athens and a combined army from Corinth and Potidaea, along with their various allies. Potidaea was a colo, a Corinthian colony, under siege. The Athenian Empire also levied economic sanctions against MegaraMegara is an ancient city in Attica, Greece, on the Saronic Gulf opposite the island of Salamis, which belonged to Megara in archaic times, before being taken by Athens. Megara was one of the four districts of Attica, embodied in the four mythic sons of K, an ally of Sparta. These sanctions, known as the Megarian decreeThe Megarian decree was the set of economic sanctions levied upon Megara by the Athenian Empire during the Peloponnesian War., were largely ignored by Thucydides, but modern economic historians have noted that forbidding Megara to trade with the prosperous Athenian empire would have been disastrous for the Megarans. The decree was likely a greater catalyst for the war than Thucydides and other ancient authors realized, more so than simple fear of Athenian power.
Sparta and its allies, with the exception of Corinth, were almost exclusively land based powers, able to summon large, usually unbeatable, land armies. The Athenian Empire, though 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. Thus, the two powers were relatively unable to fight decisive battles.
The Spartan strategy during the first war, known as the Archidamian War after its king Archidamus II, was to invade Attica, the land surrounding Athens. While this deprived Athens 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. The Spartans also occupied Attica for only a few months at a time; in the tradition of earlier hoplite warfare the soldiers expected to go home to take care of 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 Athenian strategy was initially guided by the strategos, or general, Pericles, who advised the Athenians that if they defended themselves, and did not risk offensive battles, they would be victorious. The Athenian fleet, on the other hand, went on the offensive, winning victories off Naupactus (now Návpaktos ). In 430, however, an outbreak of a plague (possibly typhus) wiped out roughly one quarter of the Athenian population, including Pericles and his sons. After the death of Pericles, his successor, Cleon, began fortifying posts around the Peloponnese, and began raiding along its coasts. One of these posts was at Pylos, where the course of the first war was decided. Cleon won a great victory at the Battle of Pylos and the related Battle of Sphacteria in 425 BC, where he captured between 300 and 400 Spartan hoplites.
The Spartans, with many of their men taken prisoner, began to press for peace; but the Athenians were now more resolute. Brasidas, a Spartan general, raised an army of allies and helots and went for one of the sources of Athenian power, capturing the Athenian colony of Amphipolis, which happened to control several nearby silver mines which the Athenians were using to finance the war. In subsequent battles, both Brasidas and Cleon were killed (see Battle of Amphipolis). The Spartans and Athenians agreed to exchange the hostages for the towns captured by Brasidas, and signed a truce.