The chronology of subsequent events is uncertain, but it is generally believed that early in 115 Trajan launched a Mesopotamian campaign, marching down towards the Taurus mountains in order to consolidate territory between the Tigris and Euphrates rivers.
He placed permanent garrisons along the way to secure the territory.
While Trajan moved from west to east, Lusius Quietus moved with his army from the Caspian Sea towards the west, both armies performing a successful pincer movement, whose apparent result was to establish a Roman presence into the Parthian Empire proper, with Trajan taking the northern Mesopotamian cities of Nisibis and Batnae and organizing a province of Mesopotamia, including the Kingdom of Osrhoene – where King Abgaros VII submitted to Trajan publicly – as a Roman protectorate.
This process seems to have been completed at the beginning of 116, when coins were issued announcing that Armenia and Mesopotamia had been put under the authority of the Roman people.
The area between the Khabur River and the mountains around Singara seems to have been considered as the new frontier, and as such received a road surrounded by fortresses.

After wintering in Antioch during 115/116  – and, according to literary sources, barely escaping from a violent earthquake that claimed the life of one of the consuls, Marcus Pedo Vergilianus – Trajan again took to the field in 116, with a view to the conquest the whole of Mesopotamia, an overambitious goal that eventually backfired on the results of his entire campaign.
According to some modern historians, the aim of the campaign of 116 was to achieve a "preemptive demonstration" aiming not toward the conquest of Parthia, but for tighter Roman control over the Eastern trade route.
However, the overall scarcity of manpower for the Roman military establishment meant that the campaign was doomed from the start.
It is noteworthy that no new legions were raised by Trajan before the Parthian campaign, maybe because the sources of new citizen recruits were already over-exploited.

As far as the sources allow a description of this campaign, it seems that one Roman division crossed the Tigris into Adiabene, sweeping south and capturing Adenystrae; a second followed the river south, capturing Babylon; Trajan himself sailed down the Euphrates from Dura-Europos – where a triumphal arch was erected in his honour – through Ozogardana, where he erected a "tribunal" still to be seen at the time of Julian the Apostate's campaigns in the same area.
Having come to the narrow strip of land between the Euphrates and the Tigris, he then dragged his fleet overland into the Tigris, capturing Seleucia and finally the Parthian capital of Ctesiphon.

He continued southward to the Persian Gulf, when, after his fleet escaped a tidal bore on the Tigris, he received the submission of Athambelus, the ruler of Charax.
He declared Babylon a new province of the Empire and had his statue erected on the shore of the Persian Gulf, after which he sent the Senate a laurelled letter declaring the war to be at a close and bemoaning that he was too old to go on any further and repeat the conquests of Alexander the Great.
Since Charax was a de facto independent kingdom whose connections to Palmyra were described above, Trajan's bid for the Persian Gulf may have coincided with Palmyrene interests in the region.
Another hypothesis is that the rulers of Charax had expansionist designs on Parthian Babylon, giving them a rationale for alliance with Trajan.
The Parthian summer capital of Susa was apparently also occupied by the Romans.

According to late literary sources (not backed by numismatic or inscriptional evidence) a province of Assyria was also proclaimed, apparently covering the territory of Adiabene.
Some measures seem to have been considered regarding the fiscal administration of Indian trade – or simply about the payment of customs (portoria) on goods traded on the Euphrates and Tigris.
It is possible that it was this "streamlining" of the administration of the newly conquered lands according to the standard pattern
The chronology of subsequent events is uncertain, but it is generally believed that early in 115 Trajan launched a Mesopotamian campaign, marching down towards the Taurus mountains in order to consolidate territory between the Tigris and Euphrates rivers. He placed permanent garrisons along the way to secure the territory. While Trajan moved from west to east, Lusius Quietus moved with his army from the Caspian Sea towards the west, both armies performing a successful pincer movement, whose apparent result was to establish a Roman presence into the Parthian Empire proper, with Trajan taking the northern Mesopotamian cities of Nisibis and Batnae and organizing a province of Mesopotamia, including the Kingdom of Osrhoene – where King Abgaros VII submitted to Trajan publicly – as a Roman protectorate. This process seems to have been completed at the beginning of 116, when coins were issued announcing that Armenia and Mesopotamia had been put under the authority of the Roman people. The area between the Khabur River and the mountains around Singara seems to have been considered as the new frontier, and as such received a road surrounded by fortresses.
After wintering in Antioch during 115/116  – and, according to literary sources, barely escaping from a violent earthquake that claimed the life of one of the consuls, Marcus Pedo Vergilianus – Trajan again took to the field in 116, with a view to the conquest the whole of Mesopotamia, an overambitious goal that eventually backfired on the results of his entire campaign. According to some modern historians, the aim of the campaign of 116 was to achieve a "preemptive demonstration" aiming not toward the conquest of Parthia, but for tighter Roman control over the Eastern trade route. However, the overall scarcity of manpower for the Roman military establishment meant that the campaign was doomed from the start. It is noteworthy that no new legions were raised by Trajan before the Parthian campaign, maybe because the sources of new citizen recruits were already over-exploited.
As far as the sources allow a description of this campaign, it seems that one Roman division crossed the Tigris into Adiabene, sweeping south and capturing Adenystrae; a second followed the river south, capturing Babylon; Trajan himself sailed down the Euphrates from Dura-Europos – where a triumphal arch was erected in his honour – through Ozogardana, where he erected a "tribunal" still to be seen at the time of Julian the Apostate's campaigns in the same area. Having come to the narrow strip of land between the Euphrates and the Tigris, he then dragged his fleet overland into the Tigris, capturing Seleucia and finally the Parthian capital of Ctesiphon.
He continued southward to the Persian Gulf, when, after his fleet escaped a tidal bore on the Tigris, he received the submission of Athambelus, the ruler of Charax. He declared Babylon a new province of the Empire and had his statue erected on the shore of the Persian Gulf, after which he sent the Senate a laurelled letter declaring the war to be at a close and bemoaning that he was too old to go on any further and repeat the conquests of Alexander the Great. Since Charax was a de facto independent kingdom whose connections to Palmyra were described above, Trajan's bid for the Persian Gulf may have coincided with Palmyrene interests in the region. Another hypothesis is that the rulers of Charax had expansionist designs on Parthian Babylon, giving them a rationale for alliance with Trajan. The Parthian summer capital of Susa was apparently also occupied by the Romans.
According to late literary sources (not backed by numismatic or inscriptional evidence) a province of Assyria was also proclaimed, apparently covering the territory of Adiabene. Some measures seem to have been considered regarding the fiscal administration of Indian trade – or simply about the payment of customs (portoria) on goods traded on the Euphrates and Tigris. It is possible that it was this "streamlining" of the administration of the newly conquered lands according to the standard pattern
Trajan's Parthian campaign