In the 8th century, a document was written that had a lasting impact on the course of medieval Europe. The Donation of Constantine granted a large amount of Roman Empire land and power to Pope Sylvester I and his successors. It was a fake.
On Christmas Day in 800 AD, Charlemagne became the emperor of Rome in a coronation headed by none other than Pope Leo III. Learn more about the growth of the Holy Roman Empire in this podcast from HowStuffWorks.com.