On the outbreak of the civil war in 49 BC, Deiotarus naturally sided with his old patron and ally Pompey and the Optimates in their fight against Julius Caesar, who was defying the Senate.
[citation needed] During this time, Pharnaces, the son of Mithridates, had seized Lesser Armenia, and defeated Deiotarus' Galatian legions which were part of Domitius Calvinus' army in the Battle of Nicopolis.
[5] In consequence of the complaints of certain Galatian princes, Deiotarus was deprived of part of his dominions in 47 BC, but was pardoned by Caesar and allowed to retain the title of King.
Other tetrarchs also pressed their claims; and, further, Deiotarus was accused at Rome in 45 BC by his grandson Castor of having attempted to assassinate Caesar when the latter, then Dictator, was his guest in Galatia.
Cicero, who entertained a high opinion of Deiotarus, whose acquaintance he had made when governor of Cilicia, undertook his defence, the case being heard in Caesar's own house at Rome.
When civil war again broke out, Deiotarus was persuaded to support the anti-Caesarian party of Brutus and Cassius, but after the Battle of Philippi in 42 BC, went over to the triumvirs.