Which emperor expanded the Roman Empire to its greatest territorial extent? (r. 98-117 AD)

The story behind the answer

Trajan (r. 98–117) expanded Rome to its greatest size. He conquered Dacia (modern Romania), annexed the Nabataean kingdom as Arabia Petraea, and launched major eastern campaigns against Parthia—capturing Ctesiphon, reaching the Persian Gulf, and briefly turning Armenia and Mesopotamia into Roman provinces. Around 117, the empire’s borders were at their widest.

By contrast, Hadrian reversed some of these gains to consolidate and fortify (think Hadrian’s Wall), Augustus built the imperial system rather than pushing maximum borders, and Nerva’s short reign was transitional. Memory aid: “Trajan’s Trajectory is outward,” while “Hadrian Holds” and “Augustus Arranges.” Trajan’s conquests also bankrolled iconic monuments like Trajan’s Forum and the famous spiral reliefs of Trajan’s Column.