The essence of what happened is that after almost a century of peace between Rome and Parthia, interrupted by a good deal of posturing, and a limited war under Nero fought over succession to the throne of Armenia, Trajan launched a grand attack into Mesopotamia. It is not clear why he did this. The political situation in Parthia appears to have been chaotic, with several rivals battling to become king of kings. Friction began in the kingdoms allied to both Rome and Parthia, most notably yet another disputed succession to Armenia. Trajan intervened, but the time and effort taken to gather so many legions in the theatre suggest that from the start he was considering a larger campaign. Some would like to see an emperor desperate for glory, perhaps even an ageing man trying to recapture the vigour of youth. Eventually, following the Tigris and Euphrates, he captured the great Parthian cities of Ctesiphon and Seleucia, and reached the Persian Gulf. The emperor was said to have watched a ship setting sail for India and wept because he was too old to match Alexander the Great and reach that distant land.

Whatever his initial intentions, Trajan overran a good deal of territory, creating a number of new provinces, and installed a friendly candidate as king of kings. For all the wishful talk of Alexander, there is no sign that he planned to destroy or take over the Parthian empire, which was still left with the bulk of its territory. Even so, this was not to be. In AD 116 rebellions broke out in many parts of the conquered territory. This was partly a natural reaction to foreign occupation, although it is also quite possible that the outbreaks were coordinated, or at least encouraged by one of the remaining rivals to the king of kings. To make matters more difficult for the Romans, the Jewish population of Egypt, Cyrenaica and Cyprus rebelled in a war that was noted for its ferocity. Stretched too thinly, and with an emperor whose health was now failing badly, the Romans suffered defeats and began to retreat. Trajan tried and failed to capture the desert city of Hatra. In AD 117 he was beginning his journey back to Rome when he fell ill and died. Hadrian succeeded him, in circumstances we shall discuss later, and abandoned most, but not all, of the conquered territory. No major aggressive wars were launched during his reign, and several frontiers saw a degree of fortification and entrenchment, most notably the building of Hadrian’s Wall, that can be seen as indicating a more defensive posture. In the longer run, there would be another major conflict when the Parthians attacked the Roman provinces at the very end of the reign of Antoninus Pius and twice under Septimius Severus a generation later. In each case the fighting shifted in favour of the Romans, who again took Ctesiphon and Seleucia. After each conflict the Romans kept control of more territory. The senator and historian Dio Cassius argued that the major gains under Septimius Severus were provocative and actually made the frontier more rather than less vulnerable.