Final answer:
Athens had a stronger navy than Sparta at the start of the Peloponnesian War. Sparta later built a significant navy with Persian aid and defeated Athens at the Battle of Aegospotami, leading to the eventual Athenian surrender.
Step-by-step explanation:
At the beginning of the Peloponnesian War, Athens possessed a stronger navy than Sparta. The Athenians, having invested in a powerful naval force, controlled the coasts and islands of the Aegean Sea. Their navy comprised hundreds of warships known as triremes, which were instrumental in maintaining Athenian military and economic strength. On the other hand, the Spartans, renowned for their prowess in land battles, initially lacked a naval force capable of challenging Athens at sea.
However, as the war progressed, Sparta realized the need to counter the Athenian navy to gain an edge in the conflict. With Persian financial aid, they built a large fleet and ultimately defeated the Athenian navy at the Battle of Aegospotami in 405 BCE. The naval defeat was a critical factor that led to Athens being under siege and unable to secure supplies, culminating in the city's surrender to Sparta and the end of the war.