History of the Peloponnesian War

As between the Athenians and the Peloponnesians, then, these additional grounds of complaint had arisen on either side, the Corinthians being aggrieved because the Athenians were besieging Potidaea, a colony of theirs with men in it from Corinth and the Peloponnesus, the Athenians, because the Peloponnesians had brought about the revolt of a city that was an ally and tributary of theirs, and then had come and openly fought with the Potidaeans against themselves. As yet, however, the war had not openly broken out, but there was still a truce for in these things the Corinthians had acted only on their own authority.

But when siege was laid to Potidaea they did not take it quietly, not only because Corinthians were in the town, but also because they were in fear about the place; and they immediately summoned the allies to Lacedaemon and, once there, they proceeded to inveigh against the Athenians on the ground that they had broken the treaty and were wronging the Peloponnesus.