[4] In 114, Trajan invaded Armenia; annexed it as a Roman province and killed Parthamasiris, who had been placed on the Armenian throne by his relative, Parthian king Osroes I.

His grand schemes for Armenia and Mesopotamia were ultimately "cut short by circumstances created by an incorrect understanding of the strategic realities of eastern conquest and an underestimation of what insurgency can do".

[22] As in the case of the alimenta, scholars like Moses Finley and Paul Veyne have considered the whole idea of a foreign trade "policy" behind Trajan's war anachronistic.

[32] One can explain the campaign by the fact that for the Romans, their empire was, in principle, unlimited and that Trajan only took advantage of an opportunity to make idea and reality coincide.

[33] Finally, there are other modern historians who think that Trajan's original aims were purely military and quite modest: to assure a more defensible eastern frontier for the empire crossing Northern Mesopotamia along the course of the Khabur River to offer cover to a Roman Armenia.

[39] At the same time, a Roman column under the legate Lusius Quietus, an outstanding cavalry general[40] who had distinguished himself during the Dacian Wars by commanding a unit from his native Mauretania,[41] crossed the Araxes river from Armenia into Media Atropatene and the land of the Mardians (present-day Ghilan).

[42] It is possible that Quietus's campaign had as a goal to extend the newer more defensible Roman border eastwards towards the Caspian Sea and northwards to the foothills of the Caucasus.

[43] The chronology of subsequent events is uncertain, but it is generally believed that in early 115, Trajan launched a Mesopotamian campaign and marched down towards the Taurus mountains to consolidate territory between the Tigris and Euphrates Rivers.

[49] 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,[50][51] Trajan again took to the field in 116, with a view to the conquest of the whole of Mesopotamia, an overambitious goal that eventually backfired on the results of his entire campaign.

[47] 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.

[67] However, as Trajan left the Persian Gulf for Babylon, where he intended to offer sacrifice to Alexander in the house in which he had died in 323 BC,[68] a sudden outburst of Parthian resistance, led by a nephew of the Parthian king, Sanatruces, who had retained a cavalry force, possibly strengthened by the addition of Saka archers,[69] imperilled Roman positions in Mesopotamia and Armenia, which Trajan sought to deal with by forsaking direct Roman rule in Parthia proper at least partially.

[71] Later in 116, Trajan, with the assistance of Quietus and two other legates, Marcus Erucius Clarus and Tiberius Julius Alexander Julianus,[72][71] defeated a Parthian army in a battle in which Sanatruces was killed.

After retaking and burning Seleucia, Trajan formally deposed Parthian King Osroes I and put his own puppet ruler, Parthamaspates, on the throne.

That event was commemorated in a coin that was presented as the reduction of Parthia to client kingdom status: REX PARTHIS DATUS, "a king is given to the Parthians".

[73] Trajan retreated north to retain what he could of the new provinces of Armenia, where he had already accepted an armistice in exchange for surrendering part of the territory to Sanatruces's son Vologeses[74] and Mesopotamia.

[70] Shortly afterwards, the Jews in the Eastern Roman Empire, in Egypt, Cyprus and Cyrene, the last province being probably the original trouble hotspot, rose in what probably was an outburst of religious rebellion against the local pagans.

[74][81] In contrast, the next prominent Roman figure in charge of the repression of the Jewish revolt, the equestrian Marcius Turbo, who had dealt with the rebel leader from Cyrene, Lukuas,[82] retained Hadrian's trust and eventually became his Praetorian Prefect.

[83] As all four consulars were senators of the highest standing and so were generally regarded as able to take imperial power (capaces imperii), Hadrian seems to have decided on a pre-emptive strike against those prospective rivals.