Callias formed an alliance with Philip of Macedon against Plutarch, tyrant of Eretria, with the view of extending his authority over the whole of Euboea, a design which, according to Aeschines, he disguised as a plan for uniting in one league the states of the island and establishing a general Euboean congress based at Chalcis.
In response to the threat from Callias, Plutarch sought aid from Athens, which was granted in opposition to the advice of Demosthenes.
Aeschines ascribes his rival's support of Callias to be the result of corruption, but Demosthenes may have thought that Euboea, united under a strong government, might serve as an effective barrier to Philip's ambition.
[2] In 341 BC, the defeat by Phocion of the Macedonian party in Eretria and Oreus under Cleitarchus and Philistides, gave control over the island to Callias.
This can be deduced by comments made by Aeschines[4], who mentions a proposal by Demosthenes to confer on Callias and his brother Taurosthenes the honour of Athenian citizenship.