Bishop Kallistos, a seemingly favorite of Latins here, back when he was Timory Ware (and more Orthodox), summarized:
At the beginning of the eleventh century there was fresh trouble over the filioque. The Papacy at last adopted the addition: at the coronation of Emperor Henry II at Rome in 1014, the Creed was sung in its interpolated form. Five years earlier, in 1009, the newly-elected Pope Sergius IV sent a letter to Constantinople which may have contained the filioque, although this is not certain. Whatever the reason, the Patriarch of Constantinople, also called Sergius, did not include the new Pope’s name in the Diptychs: these are lists, kept by each Patriarch, which contain the names of the other Patriarchs, living and departed, whom he recognizes as orthodox. The Diptychs are a visible sign of the unity of the Church, and deliberately to omit a man’s name from them is tantamount to a declaration that one is not in communion with him. After 1009 the Pope’s name did not appear again in the Diptychs of Constantinople; technically, therefore, the Churches of Rome and Constantinople were out of communion from that date. But it would be unwise to press this technicality too far. Diptychs were frequently incomplete, and so do not form an infallible guide to Church relations. The Constantinopolitan lists before 1009 often lacked the Pope’s name, simply because new Popes at their accession failed to notify the east. The omission in 1009 aroused no comment at Rome, and even at Constantinople men quickly forgot why and when the Pope’s name had first been dropped from the Diptychs.
In 1054 there was a severe quarrel. The Normans had been forcing the Greeks in Byzantine Italy to conform to Latin usages; the Patriarch of Constantinople, Michael Cerularius, in return demanded that the Latin churches at Constantinople should adopt Greek practices, and in 1052, when they refused, he closed them. This was perhaps harsh, but as Patriarch he was fully entitled to act in this manner. Among the practices to which Michael and his supporters particularly objected was the Latin use of “azymes” or unleavened bread in the Eucharist, an issue which had not figured in the dispute of the ninth century. In 1053, however, Cerularius took up a more conciliatory attitude and wrote to Pope Leo IX, offering to restore the Pope’s name to the Diptychs. In response to this offer, and to settle the disputed questions of Greek and Latin usages, Leo in 1054 sent three legates to Constantinople, the chief of them being Humbert, Bishop of Silva Candida. The choice of Cardinal Humbert was unfortunate, for both he and Cerularius were men of stiff and intransigent temper, whose mutual encounter was not likely to promote good will among Christians. The legates, when they called on Cerularius, did not create a favorable impression. Thrusting a letter from the Pope at him, they retired without giving the usual salutations; the letter itself, although signed by Leo, had in fact been drafted by Humbert, and was distinctly unfriendly in tone. After this the Patriarch refused to have further dealings with the legates. Eventually Humbert lost patience, and laid a Bull of Excommunication against Cerularius on the altar of the Church of the Holy Wisdom: among other ill-founded charges in this document, Humbert accused the Greeks of omitting the filioque from the Creed! Humbert promptly left Constantinople without offering any further explanation of his act, and on returning to Italy he represented the whole incident as a great victory for the see of Rome. Cerularius and his synod retaliated by anathematizing Humbert (but not the Roman Church as such). The attempt at reconciliation left matters worse than before.
fatheralexander.org/booklets/english/history_timothy_ware_1.htm#n4
