History of the Peloponnesian War

The Athenians then and Peloponnesians had had these previous grounds of complaint against each other; the Corinthians, because Potidaea, which was a colony of their own, and men of Corinth and from the Peloponnese in it, were being besieged; the Athenians against the Peloponnesians, because they had caused the revolt of a city which was their ally and tributary, and had come and openly fought with them in conjunction with the Potidaeans. The war however had not yet positively broken out, but at present there was a suspension of hostilities; for the Corinthians had done these things on their own responsibility alone.

When, however, Potidaea was being besieged, they did not remain quiet, as they had men in it, and were alarmed for the place. And immediately they summoned the allies to Lacedaemon, and came and cried out against the Athenians, as having broken the treaty, and as injuring the Peloponnese.