Answer:
On December 25, 800, Charlemagne was crowned Emperor of the Roman People by Pope Leo III in Rome, creating the empire we call the Carolingian Empire today, but Pope Leo and Charlemagne called the Roman Empire at the time. One little problem with this is that when the West Roman Empire fell in the 5th century, the East Roman Empire did not, and continued along, all the while calling itself the Roman Empire; today we call the medieval East Roman Empire the Byzantine Empire. Empress Irene was the empress of what she called the Roman Empire, and suddenly she got news that King Charles of the Franks was calling himself emperor of the Roman Empire. Of course, she was not happy about this.