<P> In 113, Trajan embarked on his last campaign, provoked by Parthia's decision to put an unacceptable king on the throne of Armenia, a kingdom over which the two great empires had shared hegemony since the time of Nero some fifty years earlier . It's noteworthy, however, that Trajan, already in Syria early in 113, consistently refused to accept diplomatic approaches from the Parthians in order to settle the Armenian imbroglio peacefully . </P> <P> As the surviving literary accounts of Trajan's Parthian War are fragmentary and scattered, it is difficult to assign them a proper context, something that has led to a long - running controversy about its precise happenings and ultimate aims . Many modern historians consider that Trajan's decision to wage war against Parthia might have had economic motives: after Trajan's annexation of Arabia, he built a new road, Via Traiana Nova, that went from Bostra to Aila on the Red Sea . That meant that Charax on the Persian Gulf was the sole remaining western terminus of the Indian trade route outside direct Roman control, and such control was important in order to lower import prices and to limit the supposed drain of precious metals created by the deficit in Roman trade with the Far East . </P> <P> That Charax traded with the Roman Empire, there can be no doubt, as its actual connections with merchants from Palmyra during the period are well documented in a contemporary Palmyrene epigraph, which tells of various Palmyrene citizens honoured for holding office in Charax . Also, Charax's rulers domains at the time possibly included the Bahrain islands (where a Palmyrene citizen held office, shortly after Trajan's death, as satrap--but then, the appointment was made by a Parthian king of Charax) something which offered the possibility of extending Roman hegemony into the Persian Gulf itself . The rationale behind Trajan's campaign, in this case, would be one of breaking down a system of Far Eastern trade through small Semitic ("Arab") cities under Parthia's control and to put it under Roman control instead . </P> <P> In his Dacian conquests, Trajan had already resorted to Syrian auxiliary units, whose veterans, along with Syrian traders, had an important role in the subsequent colonization of Dacia . He had recruited Palmyrene units into his army, including a camel unit, therefore apparently procuring Palmyrene support to his ultimate goal of annexing Charax . It has even been ventured that, when earlier in his campaign Trajan annexed Armenia, he was bound to annex the whole of Mesopotamia lest the Parthians interrupt the flux of trade from the Persian Gulf and / or foment trouble at the Roman frontier on the Danube . </P>

Who was the senator who adopted a general as a son and began the era of the five good emperors