Causes and Triggers of the Peloponnesian War

The Geopolitical Chessboard

Before we dive into the causes, it's essential to visualize the state of the Greek world in 431 BC. Following the Persian Wars, Greece had split into two major power blocs, each centered around a dominant city-state.

This division created a bipolar world, where the actions of one power were viewed with suspicion by the other. Think of it as a prolonged cold war, with Athens and Sparta as the two rival superpowers.

The "Truest Cause": A Shift in the Balance of Power

Thucydides argued that while many immediate grievances were cited, the real, underlying reason for the war was structural. To understand his profound argument, let's turn to a lecture by the late Professor Donald Kagan, one of the foremost authorities on this conflict.

As you just heard, Thucydides states: "The truest explanation...I believe to have been the growth of the Athenians to greatness which brought fear to the Lacedaemonians [Spartans] and forced them to war."

This is the fundamental, long-term cause. Let's break it down:

1. The Growth of Athenian Power: After the Persian Wars, Athens took the lead in forming the Delian League, a naval alliance to protect the Aegean islands from Persia. Over fifty years (a period called the Pentekontaetia), Athens transformed this voluntary league into a coercive empire. They moved the league treasury from Delos to Athens, used the money to fund massive building projects like the Parthenon, and crushed any allies who tried to leave. Athens was an innovative, expansionist, and democratic sea power.

2. The Fear of the Spartans: Sparta, the traditional land-based hegemon of Greece, was the opposite: conservative, oligarchic, and inward-looking. The Peloponnesian League was designed to maintain security in their own backyard. They watched Athens's explosive growth with increasing alarm. Spartan society was built on a fragile foundation: the control of a massive population of enslaved helots who constantly threatened to revolt. Athenian power and democratic ideology represented an external threat that could also ignite their internal vulnerabilities.

To delve deeper into this dynamic, the following paper provides a concise analysis of the security dilemma that both powers faced.

In essence, a classic security dilemma emerged. Athens expanded its empire to feel more secure, but this expansion made Sparta feel less secure, prompting it to consider war.

The Immediate Triggers: A Cascade of Crises

The long-term fear and mistrust were like a powder keg. All that was needed was a spark. In the years just before 431 BC, a series of escalating crises provided just that, drawing in allies of both leagues and making war nearly impossible to avoid.

The following video from the channel Kings and Generals provides an excellent visual narrative of these complex events.

Let's review these three key triggers:

1. The Affair of Epidamnus and Corcyra (433 BC)

This is a textbook example of a local dispute escalating into a superpower confrontation.

• The Spark: A civil war breaks out in the small city of Epidamnus.
• Escalation: The democrats in Epidamnus appeal to Corinth (a major Spartan ally) for help. This angers Corcyra, Epidamnus's mother-city.
• The Great Power Problem: Corcyra, a powerful naval state that was neutral, gets into a conflict with Corinth. Fearing defeat, Corcyra asks Athens for an alliance.
• The Athenian Dilemma: The Athenians know that allying with Corinth's enemy will infuriate the Peloponnesian League. However, Corcyra has the third-largest navy in Greece. Allowing it to be absorbed by Corinth would dramatically shift the naval balance of power.
• The Result: Athens agrees to a purely defensive alliance with Corcyra. But during the subsequent Battle of Sybota, Athenian ships are forced to engage Corinthian vessels to protect Corcyra. For the first time, Athens and a major Spartan ally had come to blows.

Your background in software development might offer a useful analogy: think of it as a dependency conflict. A problem in a minor library (Epidamnus) creates a conflict between two mid-level frameworks (Corcyra and Corinth), which in turn forces the two main operating systems (Athens and Sparta) into a state of incompatibility.

2. The Potidaean Affair (432 BC)

The fight over Corcyra made Athens nervous about Corinth's influence elsewhere.

• The City: Potidaea was in an impossible position: it was a colony of Corinth but also a tribute-paying member of the Athenian Empire.
• The Athenian Ultimatum: Fearing a Corinth-inspired revolt, Athens ordered Potidaea to tear down its defensive walls and dismiss its Corinthian magistrates.
• The Revolt: Potidaea refused and revolted, receiving unofficial military support from Corinth and a promise of invasion from Sparta.
• The Result: Athens laid siege to Potidaea. Now, Athenian soldiers were actively fighting Corinthian "volunteers" and other Peloponnesian forces. The proxy war was turning hot.

3. The Megarian Decree (c. 432 BC)

This was the economic weapon.

• The Decree: Pericles sponsored a decree banning merchants from Megara (a Spartan ally located right next to Athens) from all ports of the Athenian Empire.
• The Impact: This was a devastating economic sanction that threatened to strangle Megara.
• The Result: Megara and its allies, especially Corinth, saw this as an act of open aggression and a violation of the spirit of peace. They went to Sparta and demanded action.

The Point of No Return: Sparta Votes for War

Facing intense pressure from their allies, particularly Corinth, the Spartans convened a meeting of the Peloponnesian League. After hearing the complaints against Athens, the Spartan assembly, the Apella, voted that Athens had broken the Thirty Years' Peace and that war was necessary.

Thucydides, however, reminds us of the "truest cause." While the speeches of their allies were influential, the Spartans voted for war "because they were afraid of the further growth of Athenian power." The immediate triggers were the justification, but the underlying fear was the cause.

Conclusion

Today we've unpacked the complex origins of the Peloponnesian War, seeing how it stemmed from a combination of deep structural changes and specific diplomatic failures.

Key Takeaways:

• Long-Term Cause: The primary cause, as identified by Thucydides, was the growth of Athenian imperial power and the fear this inspired in Sparta, creating an unstable bipolar system.
• Immediate Triggers: The war was sparked by a series of escalating crises that forced the hands of the great powers:
  • The Affair of Corcyra: Drew Athens into a conflict with Sparta's powerful ally, Corinth.
  • The Revolt of Potidaea: Led to direct fighting between Athenian and Corinthian-backed forces.
  • The Megarian Decree: An act of economic warfare by Athens that crippled a Spartan ally.
• These triggers gave the war party in Sparta the justification it needed to act on its long-held fears of Athenian power.

Preview of the Next Lesson:

Now that war has been declared, what happens next? In our next lesson, we will analyze the opening strategies of the two sides. We'll see how Athens, under Pericles, planned to leverage its naval power and empire, while Sparta intended to use its legendary hoplite army to force a quick and decisive victory. The clash of these two fundamentally different approaches will set the stage for the first phase of the war: the Archidamian War.