HISTORY OF THE HOLY ROMAN EMPIRE 

Holy Roman Emperor: 800
In 799, for the third time in half a century, a pope is in need of help from the Frankish king. After being physically attacked by his enemies in the streets of Rome (their stated intention is to blind him and cut out his tongue, to make him incapable of office), Leo III makes his way through the Alps to visit Charlemagne at Paderborn.
It is not known what is agreed, but Charlemagne travels to Rome in 800 to support the pope. In a ceremony in St Peter's, on Christmas Day, Leo is due to anoint Charlemagne's son as his heir. But unexpectedly (it is maintained), as Charlemagne rises from prayer, the pope places a crown on his head and acclaims him emperor.Charlemagne expresses displeasure but accepts the honour. The displeasure is probably diplomatic, for the legal emperor is undoubtedly the one in Constantinople. Nevertheless this public alliance between the pope and the ruler of a confederation of Germanic tribes now reflects the reality of political power in the west. And it launches the concept of the new Holy Roman Empire which will play an important role throughout the Middle Ages.
The Holy Roman Empire only becomes formally established in the next century. But it is implicit in the title adopted by Charlemagne in 800: 'Charles, most serene Augustus, crowned by God, great and pacific emperor, governing the Roman empire.'Emperors and popes: 962-1250
The imperial role accorded by the pope toCharlemagne in 800 is handed on in increasingly desultory fashion during the 9th century. From 924 it falls into abeyance. But in 962 a pope once again needs help against his Italian enemies. Again he appeals to a strong German ruler.
The coronation of Otto I by pope John XII in 962 marks a revival of the concept of a Christian emperor in the west. It is also the beginning of an unbroken line of Holy Roman emperors lasting for more than eight centuries. Otto I does not call himself Roman emperor, but his son Otto II uses the title - as a clear statement of western and papal independence from the other Christian emperor inConstantinople.Otto and his son and grandson (Otto II and Otto III) regard the imperial crown as a mandate to control the papacy. They dismiss popes at their will and instal replacements more to their liking (sometimes even changing their mind and repeating the process). This power, together with territories covering much of central Europe, gives the German empire and the imperial title great prestige in the late 10th century.
But subservience was not the papal intention in reinstating the Holy Roman Empire. A clash is inevitable.Papal decline and recovery: 1046-1061
The struggle for dominance between emperor and pope comes to a head in two successive reigns, of the emperors Henry III and Henry IV, in the 11th century. The imperial side has a clear win in the first round.
In 1046 Henry III deposes three rival popes. Over the next ten years he personally selects four of the next five pontiffs. But after his death, in 1056, these abuses of the system bring a rapid reaction. Pope Nicholas II, elected in 1058, initiates a process of reform which exposes the underlying tension between empire and papacy.In 1059, at a synod in Rome, Nicholas condemns various abuses within the church. These include simony (the selling of clerical posts), the marriage of clergy and, more controversially, corrupt practices in papal elections. Nicholas now restricts the choice of a new pope to a conclave of cardinals, thus ruling out any direct lay influence. Imperial influence is his clear target.
In 1061 the assembled bishops of Germany - the emperor's own faction - declare all the decrees of this pope null and void. Battle is joined. But meanwhile the pope has been enlisting new allies.In 1059 Nicholas II takes two political steps of a kind, unusual at this period, which will later be commonplace for the medieval papacy. He grants land, already occupied, to recipients of his own choice; and he involves those recipients in afeudalrelationship with the papacy, or the Holy See, as the feudal lord.
This time the beneficiaries are the Normans, who are granted territorial rights in southern Italy and Sicily in return for feudal obligations to Rome. The pope, in an overtly political struggle against the German emperor, is playing a strong hand. The issue will be brought to a head within a few years by another pope, Gregory VII.Gregory VII and investiture: 1075
Pope Gregory seizes political control by decreeing, in 1075, that no lay ruler may make ecclesiastical appointments. Powerful bishops and abbots are henceforth to be pope's men rather than emperor's men.
The issue becomes known as the investiture controversy, being in essence a dispute over who has the right to invest high clerics with the robes and insignia of office.The appointment of bishops and abbots is too valuable a right to be easily relinquished by secular rulers. Great feudal wealth and power is attached to these offices. And high clerics, as the best educated members of the medieval community, are important members of any administration.
In subsequent periods compromises are made on both sides, particularly in the Concordat of Worms, in 1122, where a distinction is made between the spiritual and secular element in clerical appointments. But investiture remains a bone of contention between the papacy and lay rulers - not only in the empire, after the first dramatic flare up between Gregory and Henry IV, but also in Franceand England.Rome and the struggle for power: 1076-1138
The nine-year struggle between pope Gregory VII and the emperor Henry IV provides a vivid glimpse of the political role of the medieval papacy. St Gregory, canonized in theCatholic Reformation, is one of the great defenders of papal power. His career involves incessant power-broking and military struggle.
Henry IV, alarmed at the demands being made over investiture, sends a threatening letter to the pope in 1076. The pope responds by excommunicating the emperor. By his public penance atCanossa, Henry has the excommunication lifted. But the truce is short-lived. Henry's enemies, prompted by the pope's action, take a hand.German princes opposed to Henry IV elect and crown, in 1077, a rival king - Rudolf, the duke of Swabia. Rudolf and Henry engage in a civil war, which Henry wins in 1080. By then the pope has recognized Rudolf as the German king and has again excommunicated Henry.
This time Henry's response is more aggressive. He summons a council which deposes the pope and elects in his place the archbishop of Ravenna (as pope Clement III). Henry marches into Italy, enters Rome and is crowned emperor by this pope of his own creation. Meanwhile the real pope, Gregory, is living in a state of siege in his impregnable Roman fortress, the Castel Sant'Angelo.Gregory appeals for help to his vassals theNormans, recently invited by the papacy to conquer southern Italy and Sicily. A Norman army reaches Rome in 1084, drives out the Germans and rescues Gregory. But the Norman sack of the city is so violent, and provokes such profound hostility, that Gregory has to flee south with his rescuers. He dies in 1085 in Sicily.
Clement III returns to Rome and reigns there with imperial support as pope (or in historical terms as antipope) for most of the next ten years. Urban II, the pope who preaches the firstcrusade in 1095, is not able to enter the holy city for several years after his election. Unrest prevails in Rome, and uncertainty in the empire, until the Hohenstaufen win the German crown in 1138.

Information taken from: https://www.historyworld.net/wrldhis/plaintexthistories.asp?historyid=aa35