Diodotus was originally from Casiana, a dependent town of the city of Apamea.
He served as a general for Alexander Balas, during the civil war which the latter fought with Demetrius II Nicator.
In 145 BC, when Alexander's father-in-law Ptolemy VI of Egypt switched his support to Demetrius II and invaded Syria, Diodotus and a man called Hierax were in command of the city of Antioch.
They surrendered Antioch to Ptolemy and declared him king of the Seleucid Empire.
Ptolemy, unwilling to rule both the Egyptian and Seleucid realms directly, declined the title in favour of Demetrius II.

Revolt against Demetrius II
As Demetrius II secured himself on the throne, he began to eliminate former associates of Alexander Balas.
Diodotus, probably, considered himself to be in danger, and he fled to an Arab ruler called Zabdiel, who had been entrusted with the care of Alexander Balas's young son.
Diodotus with Arab support declared Alexander's son the new king, as Antiochus VI Dionysus.
The new king was less than five years old and Diodotus held all actual power as his regent.

Diodotus and Antiochus were initially ignored by Demetrius who had to consolidate his power and was also faced with financial troubles.
Utilising the discontent against the ruling regime Diodotus gathered a large army at his headquarters in Chalcis ad Belum.
Eventually, Demetrius marched against them but he was defeated in battle, after which Diodotus gained control of Apamea and Antioch.
Numismatic evidence indicates that Apamea was taken in early 144 and Antioch in late 144 or early 143.

Diodotus, in the name of the boy-king Antiochus VI, controlled most of Inland Syria, including Antioch, Apamea, Larisa and Chalcis.
Demetrius on the other hand based himself in Seleucia Pieria and retained control of many of the Syrian and Phoenician coastal cities and Cilicia.
Mesopotamia continued to recognise Demetrius as well and an invasion of the region by Diodotus in mid-144 seems to have been a failure.
Territories further east, such as Susa and Elymais were conquered by the Parthians, who eventually took control of Mesopotamia as well in mid-141 BC.

Intervention in Judaea
At the same time Diodotus made diplomatic overtures to the Jews under Jonathan Apphus in order to have them join with him against Demetrius, giving him honours and appointing his brother Simon Thassi as general.
Jonathan accepted these overtures.
Josephus justifies this by claiming that Demetrius had persecuted the Jews and that the memory of Alexander Balas encouraged them to support his son Antiochus VI.
The situation did not last, however.
The Jewish sources claim that the boldness of the Jonathan's attacks on Demetrius' supporters led Diodotus to fear his power and begin to plot against him.
In 142 BC, Diodotus dispatched troops who lured Jonathan to Ptolemais with a small guard and captured him.
Although Simon Thassi paid the money that Diodotus had demanded as a ransom for his brother, Diodotus had him executed anyway and attempted to attack Jerusalem.
Heavy snowfall forced him to abandon the siege and return to Syria.
Simon quickly became a close ally of Demetrius II, who granted extensive freedoms to him - later seen as the moment when the Hasmonean Jewish state achieved full independence.

In the same year an army of Tryphon’s routed a pro-Demetrius force under Sarpedon between Ptolemais and Tyre, but as they were marching along the coast in pursuit, a great tidal wave wiped out the army, according to Athenaeus.
Diodotus was originally from Casiana, a dependent town of the city of Apamea. He served as a general for Alexander Balas, during the civil war which the latter fought with Demetrius II Nicator. In 145 BC, when Alexander's father-in-law Ptolemy VI of Egypt switched his support to Demetrius II and invaded Syria, Diodotus and a man called Hierax were in command of the city of Antioch. They surrendered Antioch to Ptolemy and declared him king of the Seleucid Empire. Ptolemy, unwilling to rule both the Egyptian and Seleucid realms directly, declined the title in favour of Demetrius II.
Revolt against Demetrius II
As Demetrius II secured himself on the throne, he began to eliminate former associates of Alexander Balas. Diodotus, probably, considered himself to be in danger, and he fled to an Arab ruler called Zabdiel, who had been entrusted with the care of Alexander Balas's young son. Diodotus with Arab support declared Alexander's son the new king, as Antiochus VI Dionysus. The new king was less than five years old and Diodotus held all actual power as his regent.
Diodotus and Antiochus were initially ignored by Demetrius who had to consolidate his power and was also faced with financial troubles. Utilising the discontent against the ruling regime Diodotus gathered a large army at his headquarters in Chalcis ad Belum. Eventually, Demetrius marched against them but he was defeated in battle, after which Diodotus gained control of Apamea and Antioch. Numismatic evidence indicates that Apamea was taken in early 144 and Antioch in late 144 or early 143.
Diodotus, in the name of the boy-king Antiochus VI, controlled most of Inland Syria, including Antioch, Apamea, Larisa and Chalcis. Demetrius on the other hand based himself in Seleucia Pieria and retained control of many of the Syrian and Phoenician coastal cities and Cilicia. Mesopotamia continued to recognise Demetrius as well and an invasion of the region by Diodotus in mid-144 seems to have been a failure. Territories further east, such as Susa and Elymais were conquered by the Parthians, who eventually took control of Mesopotamia as well in mid-141 BC.
Intervention in Judaea
At the same time Diodotus made diplomatic overtures to the Jews under Jonathan Apphus in order to have them join with him against Demetrius, giving him honours and appointing his brother Simon Thassi as general. Jonathan accepted these overtures. Josephus justifies this by claiming that Demetrius had persecuted the Jews and that the memory of Alexander Balas encouraged them to support his son Antiochus VI. The situation did not last, however. The Jewish sources claim that the boldness of the Jonathan's attacks on Demetrius' supporters led Diodotus to fear his power and begin to plot against him. In 142 BC, Diodotus dispatched troops who lured Jonathan to Ptolemais with a small guard and captured him. Although Simon Thassi paid the money that Diodotus had demanded as a ransom for his brother, Diodotus had him executed anyway and attempted to attack Jerusalem. Heavy snowfall forced him to abandon the siege and return to Syria. Simon quickly became a close ally of Demetrius II, who granted extensive freedoms to him - later seen as the moment when the Hasmonean Jewish state achieved full independence.
In the same year an army of Tryphon’s routed a pro-Demetrius force under Sarpedon between Ptolemais and Tyre, but as they were marching along the coast in pursuit, a great tidal wave wiped out the army, according to Athenaeus.
Diodotus Tryphon