The Romans were unable to conquer the known worlds.
Battle of Sogdiana, 36 BC
Han expedition into central Asia, west of the Jaxartes River, apparently encountered and defeated a contingent of Roman legionaries. The Romans may have been the enslaved remnants of Crassus' army, defeated by the Parthians and forced to fight on their eastern frontier. Sogdiana (modern Bukhara), east of the Oxus River, on the Polytimetus River, was apparently the most easterly penetration ever made by Roman forces in Asia. The margin of Chinese victory appears to have been their crossbows, whose bolts and darts seem easily to have penetrated Roman shields and armor.
Only the Mongols were able to conquer the known worlds [annihilation of Russia, breaking it into 3 components, and conquered all the way to crush Hungary, with Crusades Knights routed along the way]: Mongol Empire was 4 times as large as the Roman Empire, and was the largest continuous empire. If anything, the Europeans should be grateful to the Mongols, for it was Timur the Great’s devastating victory against the Turks that prevented the Turks from threatening Europe again for a long time. However, the Mongols were defeated by the Ming Chinese.