History books love a clean narrative. They want to tell you that Athens and Sparta just couldn't get along, or that it was a simple "democracy versus oligarchy" grudge match. But honestly? That's not even half of it. If you want to know what started the Peloponnesian War, you have to look at a series of small, almost annoying diplomatic blunders that snowballed into a conflict that basically broke the Greek world for good.
It wasn't just one thing. It was a slow-motion train wreck.
The Thucydides Trap and the Fear of Growth
Thucydides, who actually lived through this mess and wrote the definitive account, argued that the real cause was the growth of Athenian power and the fear this caused in Sparta. This is what modern political scientists call the "Thucydides Trap."
Think about it this way. Athens wasn't just a city anymore; it was an empire. After the Persian Wars, they didn't just go home. They stayed organized under the Delian League, collected "tributes" (protection money, let’s be real) from other Greek islands, and built a massive navy. Sparta, meanwhile, was the old-school powerhouse on land. They were the established alpha dog. Seeing Athens build giant stone walls and accumulate a mountain of silver made the Spartans incredibly twitchy.
But fear doesn't start a war on its own. You need a spark. Or in this case, three or four sparks that nobody knew how to put out.
The Corcyra Incident: A Maritime Nightmare
Most people have never heard of Corcyra, but it's arguably the biggest reason the dominoes started falling. Corcyra (modern-day Corfu) had a massive navy—the third largest in Greece. They got into a nasty fight with their mother city, Corinth.
Now, Corinth was a major ally of Sparta. When Corcyra realized they couldn't beat Corinth alone, they went running to Athens. This put the Athenians in a terrible spot. If they helped Corcyra, they were essentially picking a fight with a Spartan ally, which violated the "Thirty Years' Peace" treaty they had signed. But if they did nothing, Corinth might capture Corcyra’s navy and use it to challenge Athenian sea power.
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Basically, Athens tried to "half-help." They sent a small fleet with orders to only intervene if the Corcyreans were actually being invaded. It was a classic case of trying to be a little bit pregnant. At the Battle of Sybota in 433 BCE, the Athenian ships ended up firing on Corinthian ships.
Corinth was furious. They went to Sparta and started screaming about Athenian aggression. This was the first real crack in the peace.
The Potidaea Mess
While the Corcyra drama was happening, another fire started in Potidaea. This city was weird because it was technically a member of the Athenian empire, but it was originally a colony of Corinth.
Athens, getting paranoid, told the Potidaeans to tear down their walls and expel their Corinthian magistrates. Potidaea said no. They actually revolted. Athens then sent a siege force to surround the city. Now you had Athenian troops actively fighting and killing people from a Corinthian colony.
The pressure on Sparta to "do something" was reaching a boiling point. Sparta’s allies were starting to look at them and ask, "Are you guys the leaders of Greece or just a bunch of cowards?"
The Megarian Decree: Economic Warfare
If Corcyra and Potidaea were the military sparks, the Megarian Decree was the economic gasoline. Pericles, the leading man in Athens, issued a decree that banned merchants from Megara from using any port in the Athenian Empire or the Athenian marketplace.
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Megara was an ally of Sparta. This decree was basically a total economic blockade. It was meant to starve them out or force them to switch sides.
Sparta viewed this as a direct violation of the peace treaty. They sent an ultimatum to Athens: "Lift the Megarian Decree and there will be no war." Pericles, being stubborn and perhaps a bit too confident in his "Long Walls" strategy, refused. He told the Spartans that if they wanted to talk about treaties, they should stop their own practice of "xenelasia" (expelling foreigners).
He didn't blink. Neither did they.
Why Sparta Finally Pulled the Trigger
It’s easy to blame Athens, but Sparta had internal pressures too. They had a massive population of "helots"—essentially state-owned slaves—who were always on the verge of revolting. Sparta needed to look strong. If their allies, like Corinth and Megara, felt that Sparta wouldn't protect them, those allies might leave the Peloponnesian League.
If the League dissolved, Sparta was toast.
In 432 BCE, a congress of the Peloponnesian League met. The Corinthians gave a blistering speech. They called the Spartans "un-innovative" and "slow." They warned that if Sparta didn't act now, Athens would soon be unstoppable. The Spartan assembly voted. They decided Athens had broken the treaty.
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War was declared.
Misconceptions: Was it really about Democracy?
People love to frame this as a war for "freedom." It really wasn't. Athens was a democracy, sure, but they were also an imperialist bully that forced other Greeks to pay for their fancy temples. Sparta was an oligarchy, but they actually pitched themselves as the "liberators of Greece" coming to free the cities from the Athenian yoke.
It was a war of interests, not just ideologies.
Surprising Details Often Missed:
- The Theban Attack on Plataea: The war didn't actually start with a grand battle between Athens and Sparta. It started when Thebes (a Spartan ally) tried to sneak into the city of Plataea (an Athenian ally) in the middle of the night and take it over. It was a botched commando raid that ended in a bloodbath.
- The Oracle of Delphi: Both sides were obsessed with what the gods thought. Sparta sent to the Oracle, and the word back was that Apollo would help them "whether called upon or not." That gave the Spartans the moral green light they needed.
- Pericles' Gamble: Pericles actually wanted the war to happen sooner rather than later because he thought Athens was at its peak. He didn't realize a plague would hit the city a couple of years later and kill him, along with a third of the population.
Actionable Insights for History Buffs and Students
If you're trying to wrap your head around what started the Peloponnesian War, don't just memorize dates. Look at the mechanics of how it happened.
- Analyze the Treaties: Look up the "Thirty Years' Peace" signed in 446 BCE. Understanding what the rules were makes it clear why the Megarian Decree was such a middle finger to the status quo.
- Study the Maps: Look at where Corcyra, Potidaea, and Megara are located. You'll see they are all "choke points." This wasn't about land; it was about controlling the trade routes and the sea.
- Read Thucydides, but be Skeptical: He’s the main source, but he was an Athenian general who got exiled. He had a perspective. Read Donald Kagan’s "The Outbreak of the Peloponnesian War" for a more modern, nuanced take that challenges Thucydides on some points.
- Connect it to Today: Watch how rising powers and established powers interact in modern geopolitics. The "Thucydides Trap" is still a term used in the State Department today for a reason.
The war lasted 27 years. It ended with Athens stripped of its empire, its walls torn down, and the Golden Age of Greece effectively over. All because of a few tiny disputes over colonies and trade rights that spiraled out of control. It’s a sobering reminder that once the gears of war start turning, they are almost impossible to stop.
To get a better sense of the scale, you should look into the specific military strengths of each side—Athens’ navy versus Sparta’s hoplites—to see why both sides thought they could win a quick victory.