Which historical figure is associated with the expansion of the Holy Roman Empire?

The expansion of the Holy Roman Empire is most notably associated with Charlemagne, also known as Charles the Great. Charlemagne reigned from 768 to 814 and played a crucial role in the unification of much of Western Europe under his rule. He expanded the Frankish state, which later became the foundation for the Holy Roman Empire, through a series of military campaigns that included the conquests of various Germanic tribes and the incorporation of much of Italy. In 800, Pope Leo III crowned Charlemagne as Emperor, marking the revival of the title in Western Europe and the formal recognition of his expansive rule over the territories that would later comprise the Holy Roman Empire.

Others mentioned in the question, such as Julius Caesar, Alexander the Great, and Genghis Khan, were also prominent figures known for their military conquests and expansions of territory in different contexts and regions, but their efforts did not directly relate to the Holy Roman Empire. Julius Caesar was primarily focused on the expansion of the Roman Republic, Alexander the Great expanded the Macedonian Empire, and Genghis Khan is known for creating the Mongol Empire. Thus, Charlemagne stands out as the historical figure closely associated with the expansion specifically of the Holy Roman Empire.