History of the Peloponnesian War
Thucydides
Thucydides, Vol. 1-4. Smith, Charles Foster, translator. London and Cambridge, MA: Heinemann and Harvard University Press, 1919-1923.
to Argos; but delays and postponements occurred. Meanwhile, the democracy at Argos, fearing the Lacedaemonians and again courting the alliance of the Athenians, because they believed that it would be of the greatest benefit to themselves, proceeded to build long walls down to the sea, in order that, should they be cut off from the land, they might with the help of the Athenians have the advantage of importing supplies
by sea. Some of the cities in the Peloponnesus, too, were privy to their fortifying. The whole Argive people, men, women, and slaves, set to work upon the walls; and from Athens also there came to them carpenters and stone masons. So the summer ended.
The following winter, when the Lacedaemonians became aware that they were fortifying Argos, they made an expedition thither, themselves and their allies, except the Corinthians; and there was also a party in Argos itself that was working in their interest. The commander of the army was Agis son of Archidamus, king of the Lacedaemonians.