Why did Pericles build a third Long Wall between Athens and Piraeus?

Pericles ordered the construction of a third Long Wall between Athens and its port, Piraeus, following the loss of Megara during the events of 447–446 BCE.1

The loss of Megara meant that a Spartan army could invade Attica at any time, threatening the security of the city. By constructing this additional wall, Pericles sought to improve the defensive connection between the city and its port, effectively allowing Athens to be turned into an island at will and ensuring the city could rely on its maritime empire for supplies even during a siege.12