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What caused the Peloponnesian War?

Quick answer

The Peloponnesian War (431–404 BCE) was caused primarily by the rapid growth of Athenian imperial power and the fear that expansion provoked in Sparta. After the Persian Wars, Athens transformed the mutual defense Delian League into an empire, threatening Sparta's Peloponnesian League and upsetting the balance of power across the Greek world.

The topic explained

From alliance to empire: the growth of Athenian power

The origins of the conflict trace back to the defeat of the second Persian invasion in 479 BCE. In 478 BCE, Greek city-states formed the Delian League on the island of Delos to fund a shared navy and protect against future Persian attacks. While Sparta retreated into its traditional land-based isolation in the Peloponnese, Athens supplied the admirals and the majority of the warships.

Over the next thirty years, Athenian leadership hardened into imperial rule. Member states that wished to withdraw, such as Naxos in 471 BCE and Thasos in 465 BCE, were crushed by Athenian ships and forced to pay tribute. In 454 BCE, the league treasury was moved from Delos directly to Athens, where the statesman Pericles used allied funds to build the Parthenon and finance Athenian civic payrolls. By 440 BCE, what had begun as a voluntary defensive partnership was an Athenian naval empire commanding over two hundred tributary city-states.

The clash of leagues and immediate triggers

This naval expansion directly threatened Sparta and its allies in the Peloponnesian League, especially commercial cities like Corinth and Megara. When a civil war broke out in the distant city of Epidamnus in 435 BCE, Athens allied with Corcyra, a naval power hostile to Corinth. Soon after, Athens placed economic sanctions on Megara around 432 BCE (the Megarian Decree), barring Megarian merchants from ports throughout the Athenian empire.

Sparta's allies demanded action, warning the Spartan assembly that if they did not check Athenian arrogance, the Peloponnesian alliance would fracture. In 432 BCE, Sparta voted that Athens had violated the Thirty Years' Peace established in 446 BCE. In the summer of 431 BCE, Sparta's ally Thebes attacked Plataea, and Spartan King Archidamus II led an invading army into Attica, inaugurating twenty-seven years of continuous war.

Evaluating Thucydides and his historical account

The contemporary Athenian historian Thucydides distinguished between the immediate grievances and what he termed the truest cause: the growth of Athenian power and the fear it provoked in Sparta. His structural analysis argues that when a rising power challenges an established power, war becomes almost inevitable.

When evaluating Thucydides, modern historians consider his background. He served as an elected Athenian general in 424 BCE and was exiled for twenty years after failing to relieve the strategic northern city of Amphipolis from Spartan general Brasidas. Writing in exile, Thucydides had access to both Spartan and Athenian informants. While this displacement gave him critical detachment, his political focus led him to emphasize grand geopolitical forces and elite speeches, paying far less attention to economic trade disputes or internal class struggles within the competing city-states.

Free printable worksheet

Origins of the Peloponnesian War: Causes and Sources

This worksheet is for 10th-grade history students. Use historical evidence to complete each question, analyze cause-and-effect relationships, and evaluate Thucydides' historical perspective.

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Origins of the Peloponnesian War: Causes and SourcesName:

  1. Identify the defensive alliance formed in 478 BCE to protect against Persia, and name the city-state that led it.

  2. What major action in 454 BCE symbolized the Delian League's full transition into an Athenian empire?

  3. Explain how the treatment of Naxos (471 BCE) and Thasos (465 BCE) demonstrated changing Athenian policy toward its allies.

  4. Describe the economic action Athens took against the city of Megara in approximately 432 BCE and explain why Sparta cared.

  5. According to Thucydides, what was the 'truest cause' (prophasis) of the Peloponnesian War, separate from the public complaints?

  6. Read this excerpt from Thucydides: '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 [Sparta], made war inevitable.' Evaluate whether being an exiled Athenian general helped or harmed the reliability of his analysis.

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Common questions

How did the Persian Wars directly set up the Peloponnesian War?

The Persian Wars left Athens with an unmatched fleet and prestige, allowing it to lead the Delian League. As Athens monetized this fleet into an empire, Sparta felt geographically surrounded and strategically eclipsed, turning former allies into bitter rivals.

What was the difference between the Delian League and the Peloponnesian League?

The Delian League was a maritime confederacy led by Athens where allies paid regular financial tribute. The Peloponnesian League was an older, land-based confederation led by Sparta where members provided military troops during wartime but paid no regular peacetime tribute.

Why did Sparta hesitate so long before declaring war in 431 BCE?

Spartan society relied on keeping thousands of enslaved Helots under control in the Peloponnese. Long overseas campaigns risked Helot revolts at home, making Spartan leaders cautious about entering an extended offensive war.

Did trade rivalries cause the Peloponnesian War?

Trade played a major secondary role. Athens competed with Corinth for commercial routes in the Ionian Sea and penalized Megara with harbor bans. While Thucydides downplayed commercial competition, modern historians see these economic clashes as major sparks for the war.

Who eventually won the Peloponnesian War?

Sparta won in 404 BCE after securing Persian financial backing to construct a navy. Spartan commander Lysander destroyed the final Athenian fleet at Aegospotami in 405 BCE, forcing Athens to surrender and dismantle its Long Walls.

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