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.

But the Amphilochians, when this happened, placed themselves under the protection of the Acarnanians, and together they called in the Athenians, who sent to them Phormio as general with thirty ships. On the arrival of Phormio they took Argos by storm and reduced the Ambraciots to slavery, and Amphilochians and Acarnanians settled there together.

It was after this that the alliance between the Athenians and the Acarnanians was first established.