The most obvious, of course, is that Charlemagne was the first of the Holy Roman Emperors, crowned in the year 800 by Pope Leo III. That title was not used at the time, but the title was Emperor of the Romans. His empire consisted of what is today France, Switzerland, much of the Low Countries and the western and much of the central parts of Germany. The Western Frankish Kingdom (France) later split off, and the Empire extended through Northern and Eastern Europe, but that was later. In common with many medieval commanders he was not a truly great general, but he was tireless, tenacious and a great planner. His strength of personality gained him the loyalty of competent allies and secretaries. He commanded about 50 military campaigns in his lifetime.
When crowned emperor he was already King of the Franks, as son of Peppin the Short and Bertrada, Countess of Laon. He and his brother were co-rulers until his brother's death in 771. He conquered the Lombards and then Benevento, achieving control of Italy, and then seized control of Saxony by 804. He followed this by taking control of Bavaria and redistributing the land into "counties," each controlled by a count loyal to him. This was the basis of the system of fuedal government in Europe, the holding of land by local authorities who owed allegience and aid in war to an overlord who in turn owed them loyalty and justice. He kept a picked group of knight-warriors called the vassi dominici who helped him establish and maintain his authority, bringing order to the areas under his control. He also used what were termed missi dominici, paired administrators, one clergy and one military, and his instructions to them became the basis of medieval European law.
Between 791 and 795 Charlemagne had also crushed the power of the Avar, or Huns, to the east and opened the Danube Plain to colonization by the Franks. By 800 he controlled the majority of what had been the European possessions of the old Roman Empire. Due to his political abilities and his military accomplishments, he extended his power over vast areas and at the same time brought prosperity and order with him. His campaigns also stopped the expansion of Islamic military adventurism in Western Europe. His influence in the Church brought needed reforms, and his patronage of culture and art were invaluable in the development of medieval European culture. His decrees to his officials became the basis of law throughout what is today Western and Central Europe. Not only did his efforts influence the legal and cultural basis of European culture, but his emphasis on an educated priesthood of competent administrators was influential in both church and secular government. He founded schools, including at Aachen, his home, where he died in 814. His descendants became the three families eligible for election to the post of Holy Roman Emperor up to modern times, the Wittelsbachs, Luxembourgs and Hapsburgs.