Final answer:
Charlemagne was the one who defended Pope Leo III and the Papal States. He was crowned Holy Roman Emperor by the Pope in 800, strengthening his rule over an expansive empire and setting a precedent for the emperor-papacy relationship.
Step-by-step explanation:
The individual who defended Pope Leo III and the Papal States was Charlemagne. Charlemagne, also known as “Charles the Great,” became one of the most influential rulers in the early European Middle Ages. His father, Pépin, had previously established a favorable relationship with the papacy, which Charlemagne continued. In 800, Charlemagne was crowned Holy Roman Emperor by Pope Leo III after he had earlier restored Leo to his throne amidst opposition from powerful Roman families. This event not only signified Charlemagne's domination over Leo but also strengthened his power across his empire which spanned from modern France to parts of Germany and northern Italy.
Charlemagne's coronation by the pope set a precedent for the intricate relationship between emperors and the papacy, mixing both cooperation and conflict as each saw advantages in working together against mutual enemies. Charlemagne's reign contributed significantly to shaping the political and religious landscape of medieval Europe, particularly through the concept of the Holy Roman Empire, which persisted in various forms until 1806.