…a second follows the river south, capturing Babylon; while …

…Trajan himself sails down the Euphrates, then drags his fleet overland into the Tigris, capturing Seleucia and finally, the Parthian capital of Ctesiphon.

Trajan continues southward to the Persian Gulf, receiving the submission of Athambelus, the ruler of Charax, whence he declares Babylon a new province of the Empire, sends the Senate a laureled letter declaring the war to be at a close and bemoaning that he is too old to go on any further and repeat the conquests of Alexander the Great.

A province of Assyria is also proclaimed, apparently covering the territory of Adiabene, as well as some measures seem to have been considered about the fiscal administration of the Indian trade.

However, as Trajan leaves the Persian Gulf for Babylon—where he intends to offer sacrifice to Alexander in the house where he had died in 323 BCE—a sudden outburst of Parthian resistance, led by a nephew of the Parthian king, Sanatruces, imperils Roman positions in Mesopotamia and Armenia, something Trajan seeks to deal with by forsaking direct Roman rule in Parthia proper, at least partially.

The emperor, after defeating a Parthian army in a battle where Sanatruces is killed and retaking Seleucia in 116, formally deposes the Parthian king Osroes I later in the year.

Parthamaspates, a pro-Roman son of Osroes who has spent much of his life in Roman exile, has accompanied Trajan on the latter's campaign to conquer Parthia.

Trajan had originally planned to annex Parthia as part of the Roman Empire, but ultimately decides instead to place Parthamaspates on his father's throne as a Roman client, doing so in 116.

That done, he retreats north in order to retain what he can of the new provinces of Armenia and Mesopotamia.

It is at this point that Trajan's health starts to fail him.

The fortress city of Hatra, on the Tigris in his rear, continues to hold out against repeated Roman assaults.

He is personally present at the siege and it is possible that he suffered a heat stroke while in the blazing heat.

Shortly afterwards, the Jews inside the Roman Empire rise up in rebellion once more, as do the people of Mesopotamia.

Trajan is forced to withdraw his army in order to put down the revolts.

Trajan sees it as simply a temporary setback, but he is destined never to command an army in the field again, turning his Eastern armies over to the high ranking legate and governor of Judaea, Lucius Quietus, who in early 116 had been in charge of the Roman division that had recovered Nisibis and Edessa from the rebels.

Trajan reconquers Nisibis (Nusaybin in Turkey), …

…Edessa, the capital of Osroene, a small Roman client state between Asia Minor and Syria, …

…Seleucia on the Tigris (Iraq), each of which houses large Jewish communities.

Osroes, following Roman withdrawal from the area, easily defeats Parthamaspates and reclaims the Parthian throne.

Trajan grows ill early in 117 and sets out to sail back to Italy.

His health declines throughout the spring and summer of the year, something publicly acknowledged by the fact that a bronze bust displayed at the time in the public baths of Ancyra show him clearly aged and emaciated.

By the time he reaches Selinus in Cilicia, which is afterwards called Trajanopolis, he suddenly dies from edema on August 9.

Some say that he had adopted Hadrian as his successor, but others that it was his wife Pompeia Plotina who hired someone to impersonate him after he had died.

Parthamaspates, after his defeat in Parthia, again flees to the Romans who then, as a consolation, grant him the co-rule of a restored Osroene, where he will reign together with Yalur from 118 to 122, and afterwards as sole ruler to 123.

For a century, Osroene will retain a precarious independence as a buffer state, sandwiched between the two empires.