In 116 Trajan organized the Roman conquests in Parthia and Araba into the province of Mesopotamia and (less certainly) annexed Adiabene as the province of Assyria. To celebrate the creation of Mesopotamia as a province, Trajan had coins minted; the absence of such coins for Assyria has led some experts to question its existence. In any case, both provinces lasted less than two years before being evacuated by Hadrian.
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In 116 Trajan launched a new campaign, completing his conquest of Adiabene by building a pontoon bridge over the Euphrates and capturing Arbela. He then crossed into the Parthian Empire itself, advancing in two columns down the Euphrates and Tigris to easily take Babylon and the Parthian capital of Ctesiphon. Shah Osroes withdrew his court to the ancient Persian city of Susa, but this too fell to the Roman offensive later the same year.
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Almost as soon as the Romans had conquered Parthian Mesopotamia (116), King Attambelos VII of Characene (a.k.a. Mesene) submitted to Trajan. The emperor traveled downstream to Charax Spasinu (north of Basra) and briefly sailed out on to the Erythraean Sea (Persian Gulf / Indian Ocean), where he lamented not being able to match Alexander the Great by continuing to India. Characene remained loyal to Rome throughout the Parthian rebellions, which may have led to its temporary extinguishment as a kingdom (c.118–c.131) when the Parthians regained their empire.
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When Trajan returned north to Babylon from Characene, he found that, in his absence, Edessa (capital of Osroene), Hatra (capital of Araba), and a number of Parthian cities including Seleucia had risen up in revolt against the Roman occupation. The uprisings were a combination of Parthian resistance under Shah Osroes I’s brother Mithridates, Parthian resistance backed by the rival Shah Vologases III, Jewish resistance in conjunction with the Kitos War, and local resistance. Mobilizing three task forces, Trajan eventually crushed or came to terms with all the rebels except for Hatra—which repelled even the emperor’s personally-led attempt at reconquest.
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