Deiotarus of Galatia (in Galatian and Greek Deiotaros, surnamed Philoromaios; ca 105 BC – 42 BC, 41 BC or 40 BC) was a Chief Tetrarch of the Tolistobogii in western Galatia, Asia Minor, and a King of Galatia (Gallo-Graecia) at Anatolia, Asia Minor. He was considered one of the most adept of Celtic kings, ruling the three tribes of Celtic Galatia from his fortress in Blucium. The name Deiotarus is understood as deio-taruos or "divine bull".
Deiotarus was a faithful ally of the Romans and became involved in the struggles between the Roman Generals that led to the fall of the Republic from 44 BC. He changed sides and supported the triumvirs, keeping his kingdom until his death. He is first heard of at the beginning of the Third Mithridatic War, when he drove out the troops of Mithridates VI of Pontus under Eumachus from Phrygia, and he was a witness where Mithridates VI saw his invasion of Phrygia stopped by the Roman ally, both in 75 BC or 74 BC. His most influential friend was Pompey, who, when settling the affairs of Asia after Mithridates VI failed in his invasion of Phrygia and was finally defeated by the Romans in 65 BC, 64 BC, 63 BC or 62 BC, rewarded him for his assistance against him with the title of King and an increase of territory (part of eastern Pontus, to which the Senate granted him Lesser Armenia and most of Galatia). On the outbreak of the civil war from 49 BC to 45 BC, Deiotarus naturally sided with his old patron and ally Pompey and the Optimates in their stand against Julius Caesar, who was defying the Senate, and after being defeated at the Battle of Pharsalus in 48 BC escaped with him to Asia. When Pompey was defeated he faced execution, but was saved in part due to the advocacy of Cicero between 49 BC and 45 BC. During this time, Pharnaces, the son of Mithridates, had seized Lesser Armenia, and defeated Deiotarus' army near Nicopolis. Fortunately for Deiotarus, Julius Caesar at that time (47 BC) arrived in Asia from Egypt, and was met by the tetrarch in the dress of a suppliant. Caesar pardoned him for having sided with Pompey, ordered him to resume his royal attire, and hastened against Pharnaces, whom he defeated at Zela.
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. On the death of Mithridates of Pergamum, Tetrarch of the Trocmi, Deiotarus was a candidate for the vacancy. 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. Cicero to Caius Caesar: "... for it was king Deiotarus who raised your family, when abject and obscure, from darkness into light. Who ever heard of your father, or who he was, before they heard whose son-in-law he was?" The matter was allowed to drop for a time, and the assassination of Caesar prevented any final decision of the verdict being pronounced. In his speech Cicero briefly dismisses the charge of assassination, the main question being the distribution of the provinces, which was the real cause of the quarrels between Deiotarus and his relatives. After Caesar's death, Mark Antony, for a large monetary consideration, publicly announced that, in accordance with instructions left by Caesar, Deiotarus was to resume possession of all the territory of which he had been deprived. 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. He remained in possession of his kingdom until his death at a very advanced age.
Deiotarus was the patron to whom the Greek agricultural manual by Diophanes of Nicaea was dedicated.
He was married to Berenice, Princess of Pergamon, daughter of Attalus III Philomater Euergetes, King of Pergamon, and wife Attalid (it is assumed that this link is a daughter since Attalus III is noted as dying without leaving heirs). They were the parents of Adobogiona, who married Brogitarus, King of Galatia, who reigned concurrently with his father-in-law. Adobogiona and Brogitarus were the parents of Amyntas, Tetrarch of the Trocmi, King of Galatia.
The Legio XXII Deiotariana, a Roman legion, traces its lineage back to Deiotarus. It was originally part of the Galatian Army, and the equivalent of three legions. After suffering a defeat it was considered just a single legion, and functioned as an informal Roman legion under Julius Caesar until Deiotarus' death, at which point it was incorporated into the Roman Army with its former king as its namesake.
- Cicero, Philippica, ii. 37; Ad fam. viii. 10, ix. 12, xv. I, 2, 4; Ad Att. xiv. 1; De div. i. 15, ii. 36, 37; De harusp. resp. 13, and above all Pro rege Deiotaro
- Appian, Bell. Mithrid. 75, 114; Bellum Alexandrinum, 34-41, 65-77
- Dio Cassius xli. 63, xlii. 45, xlvii. 24, 48, xlviii. 33.
- Lendering, Jona. "Legio XXII Deiotariana".
- Berresford Ellis, Peter (1998). The Celts: A History. Caroll & Graf. p. 43. ISBN 0-7867-1211-2.
- Xavier Delamarre (2003). Dictionnaire de la langue gauloise. Editions Errance, Paris, p. 142.
- "Speech in Behalf of King Deiotarus", by Marcus Tullius Cicero, translated by Charles Duke Yonge, A.B.
- Cicero. De harusp. resp. 13.
- Encyclopedia, MS Encarta 2001, under article "Galatia"
- John King, Kingdoms of the Celts.
- Christian Settipani, Les Ancêtres de Charlemagne (France: Éditions Christian, 1989).