The early period of the Christian church was troubled by a number of heresies dissensions about the nature and relationship of the three Persons of the Trinity. In the West the Holy Spirit was seen as coming from the Father and the Son, though subordinate to neither. In the Eastern part of the (as yet) undivided Catholic church the spirit was seen as coming from the Father through the Son. The phrase and the son, (in Latin, filioque), was first added to the Nicene creed at the Synod of Toledo (Spain) in 447. The formula was used in a letter from Pope Leo I to the members of that synod, responding to the trinitarian heresies they were confronting. The Eastern Orthodox churches refused to accept a formula which they saw as an innovation in doctrine. Further, although the second Ecumenical Council had amended the Nicene Creed, the third Ecumenical Council, the Council of Ephesus in 431, had forbidden any further changes to it. In the ninth century, Pope Leo III agreed to the filioque clause theologically, but was opposed to adapting it in worship in Rome, and insisted on using the Nicene Creed in Mass in Rome as it was expressed at the Council of Ephesus and all the Ecumenical Councils up until that time. The dispute, though it had not caused the severence of communion over a 600 year period, contributed to the Great Schism of the Eastern and Western branches in 1054. In addition to the actual difference in wording and doctrine, a related issue was the right of the Pope to unilaterally make a change to the Nicene Creed, as opposed to having an Ecumenical Council define the Creed.
The Roman Catholic Church has not proved unwilling to negotiate on the topic -- the Eastern rite churches of the Catholic Church -- the Maronites, the Melkites, the Ruthenians, etc. -- returned to union with the Papacy at various dates but were not required to say the "and the Son" formula in their liturgies. This may also suggest that filioque clause dispute is merely a symptom of the larger dispute concerning papal authority.