History of the Peloponnesian War

Themistocles also persuaded them to finish the walls of Piraeus, which had been begun before, in his year of office as archon; being influenced alike by the fineness of a locality that has three natural harbors, and by the great start which the Athenians would gain in the acquisition of power by becoming a naval people.

For he first ventured to tell them to stick to the sea and forthwith began to lay the foundations of the empire.