Why did Pericles tell Athenians to move behind the city’s fortified walls during Spartan invasions?

Pericles advised the Athenians to evacuate the countryside of Attica and move behind the city’s fortified walls during Spartan invasions as part of a defensive strategy to ensure Athens would "win through" the Peloponnesian War.1

This strategy was based on several key objectives:

• Maritime Security: By relying on the Long Walls, which connected the city to its port at Piraeus, Athens could effectively function as an island. This allowed the city to remain supplied by its powerful fleet even while enemy forces occupied the surrounding countryside.123
• Avoiding Land Battles: Pericles recognized that the Spartans could invade Attica at will. His strategy was largely reactive, focusing on avoiding the destruction of the Athenian empire rather than engaging the superior Spartan infantry in direct land battles.13
• Resource Management: The plan relied on utilizing accumulated capital reserves and the navy to hold the empire firmly in place, while avoiding further territorial expansion during the conflict.1

While this approach aimed to ensure survival, it faced significant political challenges, as the population deeply resented abandoning their land to Spartan ravages, and the resulting overcrowding in the city eventually contributed to a devastating plague.41