Why were Euboea and Megara strategically important to Athens?

In the context of Athenian history under Pericles, both Euboea and Megara held critical strategic value for the security and stability of Athens.1

• Euboea: This island was crucial to Athenian control of the sea and served as a vital source of food supplies for the city. Its importance was such that its revolt in 446 BCE posed a significant threat to Athenian maritime dominance and logistical stability.1
• Megara: Its strategic importance was primarily defensive. Megara acted as a buffer; when it was under Athenian influence, it helped protect the Isthmus. Its loss meant that a Spartan army could appear in Attica at any time. The vulnerability created by the loss of Megara prompted Pericles to accelerate the construction of the Long Walls, which allowed Athens to maintain its connection to the port of Piraeus and effectively function as an island.1