The declaration of Trajan as Emperor occurred after the death of Nerva, who had ruled from 96 to 98 AD. As Nerva was aware of his frail health, he adopted Trajan, a distinguished military leader and governor of Lower Germany, as his successor, indicating a shift in leadership that emphasized military success. The proclamation took place in Cologne, an important Roman city along the Rhine River, which acted as a crucial base for operations in northern territories. Trajan's rise symbolized the fulfillment of Nerva's intention to decentralize authority to capable leaders outside of Italy.

The transition of power to Trajan was marked by acclaim from both the military legions stationed in Germany and the provincial elite. He was viewed as a leader who could restore strength and vigor to a Roman Empire that had experienced turmoil and internal strife. His leadership style emphasized collaboration with the army, resulting in widespread popularity and effective governance. As Emperor, Trajan would go on to implement a series of public works projects, military campaigns, and reforms that would significantly expand Roman territory, including the successful campaigns in Dacia, which resulted in vast wealth brought to Rome.

Trajan's reign is often regarded as one of the high points of the Roman Empire, known for its expansion and relative peace, termed the Era of the Five Good Emperors. It was under his leadership that Rome achieved its maximum territorial extent, transforming the political and cultural landscapes of conquered regions. This period not only solidified Trajan's administration but also ensured that the benefits of conquest and governance reached a wider audience within the empire, marking a continuation of the complex interplay between military power and political legitimacy in Roman history.