Why do historians consider Trajan’s Parthian conquests difficult to maintain?

Historians consider Trajan’s Parthian conquests difficult to maintain primarily because they overstrained the resources of the Roman Empire and triggered widespread instability.1

Several key factors contributed to the difficulty of holding these territories:

• Unified Resistance: While the Parthian royal house had previously been divided by internal strife, the gravity of the Roman offensive forced the various princes to unite against the invader.2
• Widespread Revolts: By 116, uprisings broke out across the newly conquered territories, as well as within Jewish communities in several eastern provinces, making the region increasingly volatile.1
• Military and Logistical Strain: The campaigns were geographically vast, reaching as far as the Persian Gulf, which stretched Roman military capabilities. Some historians argue that the war was driven by a desire for military glory that ultimately exceeded the empire's capacity to sustain such distant frontiers.321
• Strategic Reassessment: Following Trajan’s death, his successor, Hadrian, concluded that the conquests in Armenia and Mesopotamia were untenable and chose to abandon them to ensure the stability of the empire.42