Introduction: The Calm Before the Storm

After decades of fighting in the First Peloponnesian War, the Greek world was exhausted. In 446 BC, the two "superpowers"—Athens and Sparta—signed the Thirty Years' Peace. This was supposed to bring stability, but instead, it became a tense countdown to the greatest conflict in Greek history: the Peloponnesian War (431 BC).

In this chapter, we will explore how a peace treaty meant to last thirty years barely made it to fifteen, and why the balance of power between these two states was so fragile. Don't worry if the names and dates seem overwhelming; we will break down the "domino effect" of events that led to war.

1. The Peace of 446 BC and the Balance of Power

The Thirty Years' Peace was a legal agreement designed to stop Athens and Sparta from clashing. Its main goal was to define spheres of influence.

Key Terms of the Peace:

  • Athens had to give up its land empire in the Peloponnese (territories like Nisaea and Pegae).
  • Both sides agreed not to interfere with the other’s allies.
  • Neutral states were free to join either side.
  • Any future disagreements were to be settled by arbitration (a legal middleman) rather than fighting.

The "Balance of Power": The peace recognized that Greece was "bipolar"—it had two poles of power. Sparta was the master of the land, and Athens was the master of the sea. As long as they stayed in their lanes, there would be peace. However, Thucydides (our main source) suggests that this balance was always under threat because Athens continued to grow in wealth and influence.

2. The Revolt of Samos (440–439 BC)

The first big test of the peace came when Samos, one of the most powerful "autonomous" allies of Athens, revolted. Samos had a large navy, so this was a major threat to the Athenian Empire.

The Role of Corinth and Sparta:

Samos asked the Peloponnesian League for help. This was a dangerous moment; if Sparta helped Samos, the peace would be broken. Thucydides and Plutarch note that the Spartans actually held a meeting to decide whether to intervene.

Why didn't they fight? Surprisingly, Corinth (a Spartan ally) argued against helping Samos. Corinth argued that every state should have the right to punish its own rebellious allies. Because of Corinth’s stance, the Peloponnesians stayed out of it, and Athens crushed the revolt.

Key Takeaway: In 440 BC, the peace held because the states still respected each other's "spheres of influence."

3. The Triggers of War: Grievances and Complaints

Between 435 and 432 BC, a series of "flashpoints" occurred. These are often called the aitiai (the complaints or grievances) that led to the outbreak of war in 431 BC.

The Corcyra Incident (435–433 BC)

Corcyra (modern-day Corfu) was a neutral state with a huge navy. They got into a fight with their mother-city, Corinth, over a colony called Epidamnus. Corcyra asked Athens for help. Athens was stuck: if they helped, they would anger Corinth (Sparta’s ally); if they didn't, Corinth might capture Corcyra’s navy and use it against Athens later.

The Decision: Athens made a "defensive-only" alliance with Corcyra. They fought Corinth at the Battle of Sybota. Corinth was furious, claiming Athens had broken the Thirty Years' Peace.

The Potidaea Affair (432 BC)

Potidaea was a weird city: it was an Athenian ally but had historical ties to Corinth (who sent them yearly magistrates). Athens, fearing Corinthian influence, ordered Potidaea to tear down its walls and kick out the Corinthian officials. Potidaea refused and revolted. Athens sent a fleet to besiege the city, further enraging Corinth.

The Megarian Decree

Athens passed a law banning the city of Megara from using any ports in the Athenian Empire. This was an economic sanction. Since Megara was a member of the Peloponnesian League, they complained to Sparta that Athens was "strangling" them.

Did you know? In his play "Acharnians", the comic poet Aristophanes jokes that the whole war started because some Megarians stole two of Pericles' prostitutes! While it's a joke, it shows how much the Megarian Decree weighed on the minds of everyday Greeks.

4. The Outbreak of War: The Debate at Sparta

In 432 BC, the Peloponnesian League met at Sparta to decide on war. This is a crucial scene in Thucydides' history.

  • The Corinthians: They insulted the Spartans, calling them slow and old-fashioned. They warned that if Sparta didn't act, Athens would soon be too powerful to stop.
  • King Archidamus (Sparta): He was cautious. He warned that Athens was wealthy and had a great navy. He suggested that "war is not a matter of arms, but of money." He wanted more time to prepare.
  • Sthenelaidas (Ephor): He gave a short, aggressive speech. He argued that Athens was hurting Sparta's allies and that "honour" demanded immediate war.

The Result: The Spartan assembly voted that Athens had broken the peace. War was declared in 431 BC.

5. The "Truest Cause" of the War

While the fights over Corcyra, Potidaea, and Megara were the immediate reasons for war, Thucydides (1.23) argues there was a much deeper reason. He calls it the "truest cause" (prophasis):

"The growth of the power of Athens, and the alarm which this inspired in Lacedaemon [Sparta], made war inevitable."

This is a vital concept for your exams. Thucydides is saying that the specific "grievances" were just excuses; the real reason was fear of Athenian expansion.

Quick Review Box:
- 446 BC: Thirty Years' Peace signed (Athens gives up land empire).
- 440 BC: Samos Revolt (Corinth prevents Sparta from helping Samos).
- 433-432 BC: Corcyra, Potidaea, and Megara "grievances" build up.
- 431 BC: War begins after the Spartans vote that the peace is broken.
- Key Concept: Thucydides' "Truest Cause"—Spartan fear of Athenian growth.

6. Summary of Sources

To do well in your AO3 (Source Analysis), remember how these authors view the period:

  • Thucydides: Focuses on power politics, the "truest cause," and the detailed debates at Sparta. He is generally seen as the most reliable, though he focuses heavily on Pericles.
  • Plutarch (Life of Pericles): Gives more personal detail about Pericles’ role in the Samos revolt and the Megarian Decree. He is writing much later, so be careful with his accuracy.
  • Aristophanes: Offers a "bottom-up" view. Through comedy, he shows how the Megarian Decree affected ordinary people and suggests Pericles used the war to distract from his own political problems.
  • Diodorus Siculus: Provides a chronological narrative but is often considered less reliable than Thucydides.

Common Mistake to Avoid: Don't just list the causes (Corcyra, Potidaea, etc.). High-level answers explain how these events connected to the Thirty Years' Peace and the "truest cause" of Spartan fear.