Up until the 12th century, papal power was very weak. The Pope himself is a monarch. The Pope is currently the monarch of The Vatican but a pope has always been the monarch of Rome/the Papal States. Individual monarchs carried power in their territories. In the medieval period, there were periods of struggle between the monarchies of Europe and papal authority. The investiture conflict in the 11th century is a great example of conflict between monarchs and papal authority in medieval Europe.

In 799, Pope Leo III was attacked in the streets of Rome. his enemies wanted to cut out his eyes and tongue to make him unfit for office. He traveled across the Alps to appeal for help from Charlemagne -- the third time in half a century that a pope would need help from the Frankish monarchy. Charlemagne traveled to Rome to have his son anointed as his heir. As claimed, Pope Leo III unexpectedly dropped a crown on top of Charlemagne's head and declared him emperor. Charlemagne "reluctantly" accepted. It was a show of alliance between the leader of some of the germanic tribes and the Pope. The Holy Roman Emperor title was not officially created until 962, when Pope John XII needed assistance from Otto I and led his coronation. The act symbolized the late Roman system of an emperor in the east (Constantinople) and an emperor in the west.

Monarchs were able to appoint bishops and other clergymen to positions within their regions -- known as lay investiture. Meaning, rulers were able to invoke spiritual appointment concerning bishops, abbots and sometimes, popes. In 1046, Holy Roman Emperor (HRE) Henry III, deposed three popes and picked four out of the five that ruled within ten years.

In 1058, Pope Nicholas II was elected and sought to execute various reforms throughout the Church. He granted land in Sicily that was already occupied, to Normans and maintained a lord-vassal relationship with them. This was normally done by monarchs rather than by papal authority and was seen as a bold move at the time.

In 1073, the investiture conflict began. HRE Henry IV claimed that newly elected Pope Gregory VII had too many enemies in high ranks of the church and could not be an effective pope. Gregory VII responded to Henry IV by excommunicating him and eliminating the support of his people. In 1075, Gregory VII decreed that no ruler could make ecclesiastical appointments and that all bishops and abbots were to answer to the Pope first. Henry appealed by public penance to the Pope at Canossa and had his excommunication lifted. Henry thought that this meant that he was king again but he was wrong. German princes failed to elect and crown Henry as emperor in 1077 and instead elected Rudolf of Swabia. Military struggle breaks out between Rudolf and Henry. After a solid victory, Pope Gregory decided to back Rudolf's claim to the throne and excommunicated Henry again. Henry pulled together a council, marched into Rome, had Pope Gregory deposed and replaced with the archbishop of Ravenna, Clement III. Gregory appealed to his Norman vassals to assist him but the sack of the city of Rome was so detrimental, that Gregory was exiled to Sicily. In 1084, Henry IV was crowned emperor by Clement III.

In 1122, the Council of Worms there was a concession that there is a secular element of ecclesiastical appointment and that there was a clear distinction between the secular and spiritual elements of that relationship.

The power of the papacy grew. When Pope Innocent III was elected, his reign was seen as the height of the papal monarchy. He excommunicated kings and declared King John of England his vassal for allowing the Magna Carta to be drafted, among other things. The struggle between investiture and excommunication, monarch and pope is a common theme throughout the Middle Ages.

Hope this helps!

Bibliography/Further Reading:

Bouchard, Constance. "˜ Feudalism, Cluny, and the Investiture Controversy." (1992).

Blumenthal, Uta-Renate. The Investiture Controversy: church and monarchy from the ninth to the twelfth century. University of Pennsylvania Press, 2010.

Miller, Maureen C. Power and the Holy in the Age of the Investiture Conflict. Bedford, 2005.

Morrison, Karl Frederick. The investiture controversy: issues, ideas, and results. Holt McDougal, 1971.

Wilson, Peter H. Holy Roman Empire. John Wiley & Sons, Ltd, 1999.