What thing began when Sparta declared war on Athens?

The Peloponnesian War began when Sparta declared war on Athens in 431 BCE. This conflict arose from escalating tensions between the two powerful city-states, primarily over issues of power, influence, and territorial disputes. The war lasted for nearly three decades, fundamentally altering the balance of power in ancient Greece and leading to significant social and political changes. Ultimately, it resulted in the decline of Athenian dominance and the rise of Sparta as a leading power.