Final answer:
Charlemagne, crowned as Emperor of the Romans, took the throne by unifying the kingdom after his brother's death and later expanding his territory and establishing himself as king over conquered lands.
Step-by-step explanation:
Charlemagne, who ascended to the throne in 768, is historically recognized for his aggressive expansion and establishment of what would eventually come to be known as the Holy Roman Empire. His father, Pepin the Short, had previously established the Carolingian dynasty by deposing the Merovingian rulers with the Pope's sanction, making the Carolingians the legitimate rulers. When Pepin passed on, the kingdom was divided between Charlemagne and his brother Carloman. After Carloman's death, Charlemagne unified the kingdom entirely under his rule, ignoring the inheritance rights of Carloman's sons, who were eventually killed. Later, Charlemagne expanded his empire by conquering other territories and establishing himself as king. He was crowned Emperor of the Romans by Pope Leo III, further cementing his power and influence over Europe.