It's good to read the whole of this article. It's a simple summary of most important history:

13. We now turn our attention to the two chief types of division as they affect the seamless robe of Christ.

The first divisions occurred in the East, when the dogmatic formulae of the Councils of Ephesus and Chalcedon were challenged, and later when ecclesiastical communion between the Eastern Patriarchates and the Roman See was dissolved.

Other divisions arose more than four centuries later in the West, stemming from the events which are usually referred to as "The Reformation." As a result, many Communions, national or confessional, were separated from the Roman See. Among those in which Catholic traditions and institutions in part continue to exist, the Anglican Communion occupies a special place.

These various divisions differ greatly from one another not only by reason of their origin, place and time, but especially in the nature and seriousness of questions bearing on faith and the structure of the Church. Therefore, without minimizing the differences between the various Christian bodies, and without overlooking the bonds between them which exist in spite of divisions, this holy Council decides to propose the following considerations for prudent ecumenical action.

Jesus, Lord of History, please give peace to the Church's Family Tree!