Many excellent historians have discussed the causes of the Peloponnesian War (431–404 BCE), and many more will do so in the future. Thucydides, however, wrote the most important contemporary chronicle of the war.

Importance of the Peloponnesian War

Fought between the allies of Sparta and the empire of Athens, the crippling Peloponnesian War paved the way for the Macedonian takeover of Greece by Philip II of Macedon and, following that, Alexander the Great's empire. Before the Peloponnesian War, the city-states (poleis) of Greece had worked together to fight off the Persians. During the Peloponnesian War, they turned on each other.

Thucydides on the Cause of the Peloponnesian War

In the first book of his history, participant-observer and historian Thucydides recorded the causes of the Peloponnesian War:

"The real cause I consider to be the one which was formally most kept out of sight. The growth of the power of Athens, and the alarm which this inspired in Lacedaemon, made war inevitable."

I.1.23 History of the Peloponnesian War

While Thucydides seemed quite certain that he had settled the question of the cause of the Peloponnesian War for all time, historians continue to debate the origins of the war. The main reasons proposed are:

Sparta was jealous of other powers and desired more power for itself.

Sparta was unhappy at no longer having all the military glory.

Athen bullied its allies and neutral cities.

There was a conflict among city-states between competing political ideologies.

Historian Donald Kagan has been studying the causes of the Peloponnesian War for decades. His 2003 book provides a detailed breakdown of the politics, alliances, and events that led to the war.

Athens and the Delian League

Many historical accounts make brief mention of the earlier Persian Wars, which undervalues their importance as a contributing factor to the later war. Because of the Persian Wars, Athens had to be rebuilt and it came to dominate its group of allies politically and economically.

The Athenian empire started with the Delian League, which had been formed to allow Athens to take the lead in the war against Persia, and wound up providing Athens with access to what was supposed to be a communal treasury. Athens used these communal funds to build up its navy and, with it, its importance and power.

Sparta's Allies

Earlier, Sparta had been the military leader of the Greek world. Sparta had a set of loose alliances by means of individual treaties that extended to the Peloponnese, excepting Argos and Achaea. The Spartan alliances are referred to as the Peloponnesian League.

Sparta Insults Athens

When Athens decided to invade Thasos, Sparta would have come to the aid of the north Aegean island, had Sparta not suffered a natural disaster. Athens, still bound by alliances of the Persian War years, tried to help the Spartans, but was rudely asked to leave. Kagan says that this open quarrel in 465 BCE was the first between Sparta and Athens. Athens broke off the alliance with Sparta and allied, instead, with Sparta's enemy, Argos.

Athens Gains an Ally and an Enemy

When Megara turned to Sparta for help in its boundary dispute with Corinth, Sparta, which was allied with both city-states, declined to come to their aid. Megara broke its alliance with Sparta and proposed a new one with Athens. Athens needed a friendly Megara on its border since it provided gulf access, so it agreed in 459 BCE. Doing so, unfortunately, set up lasting enmity with Corinth. About 15 years later, Megara joined back up again with Sparta.

Thirty Years' Peace

In 446 and 445 BCE, Athens, a sea power, and Sparta, a land power, signed a peace treaty. The Greek world was now formally divided in two, with two "hegemons." By treaty, members of one side could not switch and join the other, although neutral powers could take sides. Historian Kagan writes that, for possibly the first time in history, an attempt was made to keep the peace by requiring both sides to submit grievances to binding arbitration.

Fragile Balance of Power

A complicated, partially ideological political conflict between Spartan-ally Corinth and her neutral daughter city and strong naval power Corcyra led to Athenian involvement in Sparta's realm. Corcyra appealed to Athens for help, offering to Athens the use of its navy. Corinth urged Athens to remain neutral. But since Corcyra's navy was powerful, Athens was concerned that it would fall into Spartan hands and disrupt whatever fragile balance of power the city-states were maintaining.

Athens signed a defense-only treaty and sent a fleet to Corcyra. Fighting ensued and Corcyra, with Athens' aid, won the Battle of Sybota against Corinth in 433. Athens now knew that direct battle with Corinth was inevitable.

Spartan Promises to Athens' Ally

Potidaea was part of the Athenian empire, but also a daughter city of Corinth. Athens feared a revolt, with good reason, since the Potidaeans had secretly acquired a promise of Spartan support, to invade Athens, in violation of the 30 years treaty.

Megarian Decree

Athens' former ally, the polis Megara, had allied with Corinth at Sybota and elsewhere, and Athens, therefore, put a peacetime embargo on Megara. Historians are not clear on the embargo's effects, some saying that Megara was merely made uncomfortable, while others claim that it set the polis on the brink of starvation.

The embargo was not an act of war, but Corinth took the opportunity to urge all allies disaffected with Athens to pressure Sparta now to invade Athens. There were enough hawks among the ruling bodies in Sparta to carry the war motion. And so the full-fledged Peloponnesian War began.

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