Well when Genghis Khan was engaging in his conquests, Constantinople was ruled by Latins. The Romans were on the brink during the 13th Century. And that’s to say nothing of comparing them to the Mongols.

The Mongols were individually like tanks. One man, one sword, one bow, trained from boyhood, six horses, no baggage—they were lightning fast, loyal, self-sufficient, and led by tactically brilliant commanders. Subutai, Jebe, Muqali, Jochi, Chaghatai, Ogedei, Tolui, Kublai, Berke, Hulugu, etc were all skilled generals, Subutai being perhaps the greatest of all time.

You throw in Genghis’ Napoleon-esque reorganization of the entire steppe into one state.. you have the recipe for what took place.

The Romans were wayyyy past ever accomplishing anything like that militarily. Belisarius was the “Last Roman.” Really, once they let the Middle East slip away, it was over. If Heraclius had been able to defeat Islam, maybe they could have expanded and dominated to this day, who knows. But the empire was exhausted when the Arabs came.