Final answer:
Clovis I was the first ruler to unite many of the Germanic kingdoms by founding the kingdom of the Franks in the early sixth century. Charlemagne significantly expanded the empire later and was crowned Emperor of the Romans in 800, establishing the foundations of the Holy Roman Empire which was cemented by Otto I's coronation in 962.
Step-by-step explanation:
First Ruler to Unite Germanic Kingdoms
The first ruler to unite many of the Germanic kingdoms in the early Middle Ages was Clovis I, the Merovingian king who founded the kingdom of the Franks in the early sixth century. His unification of the Franks marked the beginning of the establishment of a lasting kingdom in the region of Gaul, present-day France. Clovis was known for his ruthless tactics and significant expansion of territory, which included defeating the last remnants of Roman power in Gaul.
However, it was Charlemagne, a later Germanic ruler, who dreamed of reviving the Roman world and significantly expanded the empire during his reign from 768 to 814, uniting western Europe for the first time since the fall of the Roman authority. He reorganized government, promoted learning, and on Christmas Day in 800, was crowned as the Emperor of the Romans by Pope Leo III, thereby establishing the roots for what became known as the Holy Roman Empire.
While Otto I, coming to power after the Carolingian dynasty, which included Charlemagne, was not the first to unite Germanic kingdoms, he did cement the concept of the Holy Roman Empire after being crowned emperor in 962, thus ensuring the continuation of a united entity in the region that Charlemagne had first consolidated.