The first Roman advance across the Euphrates ended in catastrophe in 53 BCE. The Roman triumvir Crassus sought for himself the same glory that his co-rulers Julius Caesar and Pompey had achieved. Instead, at Carrhae, south of Edessa, Rome suffered one of its most humiliating defeats, and Crassus lost his life. Three decades later Emperor Augustus and King Phraates IV ended a war neither could win and, in 20 BCE, recognized the Euphrates as the border, thus beginning the Pax Romana. The river became the place where East met West – whether in friendship or in rivalry. For the spread of Christianity and for the history of the Church of the East, this border took on a fateful significance. Although Roman armies crossed the river repeatedly in the second and third centuries and conquered Seleucia-Ctesiphon, they had to retreat every time. Politically and culturally, Mesopotamia remained part of Asia.
Even the first Christian historian, Eusebius of Caesarea (265-339) in his ecclesiastical history, devoted scarcely a word to the Asian Christianity of Mesopotamia that had begun to develop rapidly in the second century. The reasons behind this the lack of attention to the Church of the East grew out of the geopolitical situation of the time. In those days the River Euphrates, with its source in the north-east of modern Turkey and its mouth at Basra on the Persian Gulf, separated the Roman Empire from the Iranian Empire. Aside from isolated Roman advances towards the east and Iranian advances towards the west, the Euphrates stood as a stable national boundary, whose political impermeability was breached only by merchant caravans.
Baumer, The Church of the East, pp. 1-2
It’s interesting what powerful effect the Euphrates river had on the development of the early church. As a major geographic barrier, it also functioned as political, linguistic, and cultural barrier, separating civilizations and leading to a western forgetfulness, to this day, of the millions of Christians that lived outside of the Roman Empire, in the lands to the East.