History of the Peloponnesian War

the Chalcidians thinking that the Athenians would in the first place march against them, (and moreover, the cities near to them which had not revolted, secretly joined in the invitation,) and Perdiccas, though not an open enemy, yet being afraid, on his part also, because of his old quarrels with the Athenians, and most of all being desirous of reducing to subjection Arrhibaeus, the king of the Lyncestians.

And what contributed to their getting the army out of the Peloponnese the more easily, was the misfortune of the Lacedaemonians at that time. For as the Athenians were pressing hard upon the Peloponnese, and especially upon their territory, they hoped to divert them from it most effectually, if they annoyed them in return by sending an army to their allies; especially as they were ready to maintain it, and were calling them to their aid, with a view to revolting.

Besides, they were glad to have a pretext for sending away some of the Helots; lest in the present state of affairs, when Pylus was occupied by an enemy, they might attempt some revolution.