Charlemagne is credited with uniting Europe after the fall of the Roman Empire and inventing the medieval form of government. He also is credited with defeating the various Barbarian tribes from the east such as the Tartars that were threatening to overrun Europe and with re-discovering the classics of Greece and Rome and with founding the first university in Europe, which was to lead to the Carolingian Renaissance. Supporting the Pope of Rome, Charlemagne is also credited with the spread of Christianity throughout Europe. About a half of the U.S. Presidents can trace their ancestry back to Charlemagne with some, such as George Washington, able to trace their ancestry through several lines back to Charlemagne. According to the oral and written traditions of the Franks, they were descended from the Trojans and had migrated westward into the area east of the Danube and eventually further west to the areas that now include Belgium, France and Germany. One of the cities they founded was Paris, named after the famed Paris of the Trojan War.