Read Francis Dvornik's "The Photian Schism: History and Legend".
In 843, the second Iconoclast crisis ended in the East, as Ecumenical Patriarch John VII was removed by Empress Theodora. But after the death of Ecumenical Patriarch Methodius in 847, the tension between Iconodules and Iconoclasts began to spark again, and Ecumenical Patriarch Ignatius was chosen by Empress Theodora, due to his non-involvement with the matter. But he made things worse, by condemning the Archbishop of Syracuse with a synod, and this archbishop, Gregory Asbestas, appealed the case to Rome.
According to Byzantine sources, Pope Leo IV asked Ecumenical Patriarch Ignatius to send representatives to Rome to discuss the matter, and Pope Benedict III confirmed Gregory's condemnation. But according to Pope Nicolas, what happened was that Popes Leo and Benedict refused to judge it, and so Gregory's condemnation is not canonical because the Church of Rome, even though it holds the final judgement for everything, did not get a say in it. Again according to Pope Nicholas, Pope Benedict asked for Ecumenical Patriarch Ignatius to send representatives, but he did not do so. Francis Dvornik speculates that this is because Ignatius did not consider it necessary for the Pope to give his judgement on an issue that he himself, the Patriarch of Constantinople, has already confirmed.
In 858, Empress Theodora was forced out of power by her son Emperor Michael III, and Ignatius was replaced with Ecumenical Patriarch Photius, as punishment for not sanctioning the deposition of the empress. Photius was a well-learned scholar, but was elevated to the status of Patriarch in a very irregular manner, receiving the status of lector, subdeacon, priest, then bishop within 5 days. Pope Nicholas argued against the deposition of Ignatius and the elevation of Photius however, because the Church of Rome had not been notified of this and its judgement was not seeked.
In 861, the papal legates arrived in Constantinople and demanded to judge the case. Although they were told off at first (due to the Constantinopolitan bishops feeling that the judgement of Rome was unecessary and the issue was already settled), they were then allowed to do so, to honor the Apostolic See. They recognized the elevation of Photius, which greatly angered Pope Nicholas.
Pope Nicholas was further motivated by pro-Ignatius partisans in Rome who suggested to him that Ignatius would recognize papal jurisdiction over Illyricum if he were re-instated, and so, in 863, the Pope held a synod at the Lateran deposing Photius and re-instating Ignatius as Ecumenical Patriarch… except this synod would simply be ignored by Constantinople.
King Boris of the Bulgars took profit of this situation. Although the Bulgarian Church had Constantinople as its mother Church, he feared the growing hegemony of the Ecumenical Patriarchate, and therefore wrote to Pope Nicholas to ask to be put under the spiritual authority of Rome instead, with the hope that the Pope may recognize the Church of Bulgaria as a Patriarchate. King Boris ejected the Byzantine missionaries to receive Frankish ones, and the Byzantines observed the Latin practices of the Franks and reported them back to Photius. Angered by the "betrayal" of the Bulgars and the "attack" of the Franks upon Byzantine jurisdiction, Photius wote an "Encyclical to the Eastern Patriarchs", accusing the Western Church of heresy because of its non-compliance to the Quinisext Council. In response, Pope Nicholas asked Hincmar of Reims, Liutbert of Mainz, Aeneas of Paris, and Ratramnus of Corbie to write something refuting Photius's attacks on Latin Christianity and proving the orthodoxy of the West. Meanwhile, Photius (possibly) wrote the "Mystagogia of the Holy Spirit" and Niketas Byzantinos wrote the "Syllogistic Chapters".
(cont)