History of the Catholic Church
The papacy acquired a new protector in the West with the coronation of Charlemagne by Pope Leo III in 800 and the handing over of the keys to the Tomb of Saint Peter. This freed the pontiffs from the power of the emperor in Constantinople. However, it also led to a schism, because the emperors and patriarchs of Constantinople interpreted themselves as the true descendants of the Roman Empire. Pope Nicholas I had refused to recognize Patriarch Photios I of Constantinople, who in turn had attacked the pope as a heretic. This was because he kept the filioque in the creed, which referred to the Holy Spirit emanating from God the Father and the Son. The papacy was strengthened through this new alliance, which in the long term created a new problem for the Popes. This was that, in the Investiture controversy, succeeding emperors sought to appoint bishops and even future popes. After the disintegration of the Carolingian Empire and repeated incursions of Islamic forces into Italy, the papacy, without any protection, entered a phase of major weakness.