Pope Leo III crowned Charlemagne Holy Roman Emperor on Christmas Day, 800, in Rome. The event was significant for several different reasons. For Charlemagne, it meant that the Church, very powerful in early medieval Europe, gave its sanction to his rule. He had spent decades consolidating his power over much of Western Europe, including Italy, Germany, and Spain, through warfare and diplomacy. Though contemporary chroniclers claim Charlemagne did not desire to be crowned by the Pope, his coronation ceremony gave him additional legitimacy. It also enabled him, through the Church bureaucracy, to rule more effectively, as he had a hand in many appointments to bishoprics and other offices. It also made him the equal in power and stature of the Byzantine emperor in Constantinople.

For the Pope, it meant that the Catholic Church had the protection of the most powerful ruler in Europe. This was important because the Pope had long been challenged in Italy by rebellious nobles who chafed at the secular power of the papacy. He had long sought, and received, Charlemagne's support and protection. The coronation at Rome signaled that the papacy would continue to enjoy the protection of the Frankish kings. In terms of historical significance, the crowning is often held to represent a newly unified Roman Empire, though this would be fleeting. If anything, it demonstrates how the idea of a unified Western Europe under Christian rule was viewed as desirable by many Europeans.

Charlemagne continued the policy of his father, King Pepin the Short, of supporting the papacy against the Lombards (Italians in the region around Rome) and of granting the papal state land around Rome as a Frankish fiefdom. Charlemagne defended the election of Pope Leo III against enemies who wanted to reject him on the grounds he was not an aristocrat. Two days after his consecration as pope, Leo crowned Charlemagne as the Holy Roman Emperor.

This was important for several reasons. First, it signaled the importance of the political alliance between the pope and the Franks (French), showing that the papal states and the Frankish states were a powerful European bloc with wealth, military might, and the special imprimatur of holy legitimacy. Second, for the first time since the fall of Roman Empire, a Western monarch was named Roman Emperor. While this was a symbolic title, as Charlemagne ruled only a tiny fraction of the former Roman Empire, it added to his status and showed the growing extent of his power. The ability to name a ruler emperor also showed that Europe was beginning to coalesce into larger states and reestablish at least a small part of the stability that had once characterized the Roman Empire.

Further, the Eastern Orthodox Church in Constantinople was upset and disapproving of the alliance between Leo III and Charlemagne, as was Empress Irene of Athens, who thought the Eastern Roman Empire should be the protector on the Roman church, not the Franks. However, her government was too weak and corrupt to provide Leo III the support he needed.

The coronation of Charlemange by the Roman pope showed the ascendant power of Western Europe and the decline of the Eastern Roman Empire. It also signaled the the pope would become an important player in legitimizing the rule of monarchs in Western Europe.

Charlemagne, King of the Franks, was crowned as the Holy Roman Emperor on Christmas Day, 800 A.D. by Pope Leo III. The coronation was important to the Pope as it recognized how important Charlemagne had been to him in protecting him from rebels in Rome. It also meant that the Pope would be protected from the power of the rival Eastern Empire.

For Charlemagne, the coronation gave his rule religious legitimacy and meant that he was equal in power to the emperor of the Eastern Empire. As a result, a close connection between religious and secular power in Europe was forged, and this connection would remain strong until the end of the Middle Ages. In addition, Charlemagne's coronation marked the Western Empire as distinct from the Eastern Empire and fostered the growth of Europe as separate from the Eastern Empire. 

The coronation of Charles gave the Pope the power to excommunicate any person who refused to bear arms in any cause of Charles, thereby in effect giving the Vatican control over all the armies of any Christian nation.  This is the ultimate example of church/state affiliation, because the individual conflicts of two countries, whether border disputes, questions of succession, or any other problems, immediately conflated into a continent-wide, Pope-driven war, and the spiritual integrity of Pope Leo was set aside in favor of the very earth-bound desires of the powers of the Vatican, itself often orchestrated by powerful Italian families.  

The crowning of Charlemagne by Pope Leo III was important in a number of ways.  

For Charlemagne, it was important because it helped to give him greater credibility.  It gave him the status of an emperor, making him the only emperor in Europe west of the Byzantine emperor in Constantinople.  This made Charlemagne seem like the most important ruler in Europe.

For the Pope, it was important as well.  By crowning Charlemagne, Pope Leo got himself an important ally.  Leo had had to flee to Charlemagne's court just the year before when a rebellion erupted against him.  By crowning Charlemagne, Leo was connecting himself politically to Charlemagne and thus gaining some power himself.

Finally, the crowning of Charlemagne was important for Europe as a whole.  It represented the creation of the Holy Roman Empire.  This was a first step towards creating a true European society such as had not existed since the "fall" of Rome.