The final spark was to ban Megara from thading in the Athenian empire, which would ruin it. Megara appelaed to its Peloponnesian allies who gave Athens an ultimatum to lift the ban. Athens refused, and both sides were at war.
There was of course a lead up, where an-over-confident Athens had been intervening in Corcyra and Potidaia, and the Megara standoff was the final straw.
Athens won the first Peloponnesian War but Sparta won the second war
Athens and its allies were defeated by Sparta in the Peloponnesian War.
There was one Peloponnesian War. It was finally sparked by Athens' refusal to lift a ruinous trade ban on Megara.
. . . the Peloponnesian War .
Athens and Sparta
Athens and Sparta fought in the Peloponnesian War.
The city-state of Athens and its allies were defeated by Sparta in the Peloponnesian War.
Contrast the results of the Persian and Peloponnesian war with regards to Athens Greece
The Peloponnesian War
The Peloponnesian War
The Peloponnesian War was between alliances of Greek city-states - Athens and its empire, and the Peloponnesian League led by Sparta.
Sparta's declaration of war against Athens began the Peloponnesian war. Sparta accused Athens of breaking their peace and the war began in 431 BCE.