The Peloponnesian War was primarily caused by the growth of Athens' power in Greece. Athens had developed a strong economy, navy, and army, and was a major cultural center of the Greek world. This expansionist policy directly threatened the power and interests of Sparta, as well as other city-states in the Peloponnesian League.
2. Trade Disputes
Athens' expansionism had a significant impact on the trade routes, particularly in the Aegean Sea, which disrupted long standing trading partners throughout Greece. This, combined with Athens' control over the Hellespont, caused major economic conflicts with several Greek cities and their commercial interests.
3. Megara's Exclusion from the Athenian Trade Empire
Megara, a city-state strategically located on the border of Attica, was a long-standing rival of Athens. Athens implemented harsh economic sanctions on Megara, ultimately expelling it from the Delian League (an Athenian-led alliance of Greek city-states). This action directly contravened the terms of the Thirty-Year Peace between Sparta and Athens, and further strained relations between the two city-states.
4. Corcyra's Conflict and Athens' Intervention
In 433 BC, the city-states of Corcyra (on the island of Corfu) and Epidamnus (on the mainland) became involved in a conflict over the control of Epidamnus. The Corcyrans, who had a fleet superior to that of Athens, sought Athenian support in the dispute. Athens agreed to provide ships to Corcyra, which angered the Corinthians (allies of Sparta and Epidamnus). This involvement marked a further deterioration in relations between Sparta and Athens.
5. Potidaea's Revolt and Athenian Blockade
Potidaea, a Greek colony on the Chalcidice peninsula, was a tributary ally of Athens. However, its close relationship with Corinth (a Peloponnesian ally) raised suspicions of disloyalty. Athens demanded Potidaea to expel its Corinthian citizens, sever its ties with Corinth, and demolish a section of its wall. Potidaea refused, leading to an Athenian blockade and eventual siege of the city. This violation of Potidaea's sovereignty served as a direct challenge to Sparta's authority and was seen as a major provocation by the Peloponnesian League.
These major conflicts and tensions eventually led to the outbreak of the Peloponnesian War in 431 BC, which lasted for nearly three decades and had a profound impact on the Greek world.