Final answer:
The Peloponnesian War began as a commercial rivalry between Athens and Sparta in 431 BCE.
Step-by-step explanation:
The Peloponnesian War began as a commercial rivalry between Athens and Sparta. The wealth and power of Athens greatly concerned the Spartans, leading to a rivalry between the two city-states. This eventually escalated into open conflict in 431 BCE when Sparta and its allies invaded Attica, marking the start of the Peloponnesian War.