1. Rivalry between Sparta and Athens: Sparta was the dominant land power in Greece, while Athens was a powerful naval and commercial city-state. Both cities competed for influence and power in the Greek world, and their rivalry eventually led to war.
2. Athenian expansionism: Athens pursued an aggressive policy of expansion in the Aegean Sea and the Black Sea, which brought it into conflict with Sparta and its allies.
3. Megarian Decree: Athens imposed an economic embargo on Megara, a city-state allied with Sparta. This action directly harmed Spartan interests and was seen as a hostile act.
4. Corinthian Crisis: Epidamnus, a Greek colony on the Adriatic Sea, was torn by internal conflict. The people of Epidamnus called on Corinth, its mother city, for help, but Corinth turned to Sparta for assistance. Athens, allied with Corcyra, an enemy of Corinth, intervened on the side of Corcyra.
5. Thebes-Plataea Conflict: Thebes, an ally of Sparta, attacked Plataea, an ally of Athens. This further increased tensions between the two major powers.
6. Fear and Mutual Distrust: Both Sparta and Athens feared and distrusted each other. The growth of Athenian power and their democratic system worried the Spartans, while the Spartan control over the Peloponnesus and the oligarchic nature of their society concerned the Athenians.
Additional Factors:
- Clash of Ideologies: Athens, known for its democracy and progressive ideas, differed fundamentally from Sparta's conservative approach to governance and emphasis on military strength.
- Colonial Disputes: Colonies of Greek city-states spread across the Mediterranean and the Black Sea, creating overlapping spheres of influence and economic competition.
The Peloponnesian War began in 431 BC and lasted for nearly three decades, leading to profound changes in the power dynamics and political landscape of ancient Greece.