The fact that we call the war between the Delian League (Athenian Empire) and Sparta the Peloponnesian War shows that Thucydides saw the war through a veil of preconception do to his loyalty to Athens. Thucydides, an Athenian statesman and general was convinced from the outset that the war would be the most important ever recorded in Greece and therefore made great efforts to establish the exact truth. Since he was exiled early in the war for failing to relieve a besieged Athenian territory, he had plenty of freedom to travel and to talk to both Spartans and Athenians.
Warfare in Hellenic Greece centered mainly around heavy infantrymen called hoplites. They were armed as spearmen, which are relatively easy to equip and maintain. And mainly they represented the middle class, who could afford the cost of the armaments. Almost all the famous men of ancient Greece, even the philosophers and playwrights, fought as a hoplite in some battle or another. Hoplites generally armed themselves immediately before battle, since the equipment was so heavy. Each man provided his own gear so it was fairly non-uniform, and often friendly troops would fail to recognize one another. A hoplite typically had a breastplate, a bronze helmet with cheek plates, as well greaves and other armor, plus a bowl-shaped wooden shield around 1 meter across. The primary weapon was a spear, around 2.7 meters in length; as this frequently broke upon charging, hoplites also carried a smaller 60 cm thrusting sword.
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 an alliance of Greek states stopped 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 Potidaea, a Corinthian colony, under siege. The Athenian Empire also levied economic sanctions against Megara, an ally of Sparta. These sanctions, known as the Megarian decree, 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.
As the war began, Sparta and Athens each took advantage of their military strengths. Sparta, with its much larger army, ravaged Attica the territory around Athens while the Athenian navy raided cities on the Peloponnesus. This strategy lasted for two years. Meanwhile Pericles death in 429 left the democracy prey to hostile factions and reckless leaders who pursued their own advantage. Most of the leaders were warmongers who insisted on vigorous prosecution of the conflict. Chief among these select few was Alcibiades, who was as irresponsible as he was brilliant. By 425 Sparta’s hopes for victory were bleak, and its leaders were ready to ask for peace. Slowly, however, the fortunes of war changed. Sparta, under its general Brasidas, scored significant victories at Chalcidice and Amphipolis. Both were serious losses for Athens. The Athenian leader Nicias persuaded the city to accept Sparta’s offer to cease hostilities in 421. And despite Thucydides’ prejudices he had the best insight into the period and the war itself.