The Peloponnesian War - Causes of the Conflict

What Caused the Peloponnesian War?

Aegean area before the Peloponnesian war

 Wikimedia Commons

Many excellent historians have discussed the causes of the Peloponnesian War (431-404), and many more will do so, but Thucydides, who lived at the time of the war, should be the first place you look.

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 [see Philip II of Macedon] and Alexander the Great's empire. Earlier -- that is, before the Peloponnesian War -- the poleis of Greece had worked together to fight off the Persians. During the Peloponnesian War, they turned on each other.

Thucydides on the Causes of the Peloponnesian War

In the first book of his history, participant observer and historian Thucydides records the causes of the Peloponnesian War. Here is what Thucydides says on the causes, from the Richard Crawley translation:

"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 may have thought he settled the causes of the Peloponnesian War for all time, historians continue to debate the causes of the war. The main suggestions are:

  • Spartan jealousy and desire for more for itself,
  • Spartan unhappiness at no longer having all the military glory,
  • Athenian bullying of its allies and neutral cities, and
  • A conflict between some competing political ideologies.

Donald Kagan has been studying the causes of the Peloponnesian War for decades. We are relying chiefly on his analyses, mainly from his 2003 analysis. Here is a look at the situations and events that caused the Peloponnesian War.

Athens and the Delian League

Mention of the earlier Persian Wars doesn't just put the later events in a time frame. As a result of the wars [see Salamis], Athens had to be and was rebuilt. 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 it to build up its navy and therefore 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 timely 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 was the first between Sparta and Athens. Athens broke off the alliance with Sparta and allied, instead, with Sparta's enemy, Argos.

Athens Zero-Sum-Gain: 1 Ally + 1 Enemy

When Megara turned to Sparta for help in its boundary dispute with Corinth, Sparta, allied with both poleis, declined. Megara suggested that it break the alliance with Sparta and join up with Athens. Athens could use a friendly Megara on its border since it provided gulf access, so it agreed, although doing so set up lasting enmity with Corinth. This was in 459. About 15 years later, Megara joined back up again with Sparta.

Thirty Years' Peace

In 446/5 Athens, a sea power, and Sparta, a land power, signed a peace treaty. The Greek world was now formally divided in two, with 2 "hegemons". By treaty, members of one side could not switch and join the other, although neutral powers could take sides. Kagan says that for probably 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's offer included the use of her navy. Corinth urged Athens to remain neutral. Since Corcyra's navy was powerful, Athens did not want it to fall into Spartan hands and disrupt whatever fragile balance of power there was. Athens signed a defense-only treaty and sent a fleet to Corcyra. Intentions may have been good, but fighting ensued. Corcyra, with Athens' aid, won the Battle of Sybota against Corinth, in 433.

Athens now knew 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 (actually, to invade Athens), in violation of the 30 years treaty.

Megarian Decree

Megara had recently helped Corinth at Sybota and elsewhere, so Athens put a peacetime embargo on Megara. The decree would only make Megara uncomfortable, although possibly put it on the brink of starvation (Aristophanes Acharnians) without being an act of war, yet 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.

"The Causes of the Peloponnesian War," by Raphael Sealey. Classical Philology, Vol. 70, No. 2 (Apr., 1975), pp. 89-109.