Complex sentence: In 115 the emperor Trajan was in command of the eastern campaign against the Parthian Empire, which had arisen over the imposition of a pro-Parthian king on the throne of Armenia after a Parthian invasion of that land.
Split into simpler sentences: In 115 the emperor Trajan was in command of the eastern campaign against the Parthian Empire. The Roman invasion had been prompted over the imposition of a pro-Parthian king on the throne of Armenia after a Parthian invasion of that land.