Before anyone gets too excited about those links, it’s important to note that 4 of them are to articles in NewAdvent, the on-line 1911 (?) edition of the Catholic Encyclopedia, making them outdated by a century.
Now, one can argue that, as the OP asked for a historical explanation, that lapse in time is inconsequential. But, it is not. Today, other than the broad-based topic of “Ecclesiastical History”, such would most likely be contributed (one hopes) by clergy or laypersons of Eastern Christian heritage - or, at the very least, co-authored by Eastern and Western scholars. Not so in the era that brought them to paper - particularly in an English-speaking nation where:
[list]*]25 years earlier a schism had been narrowly avoided in the face of the inhospitable - no, outright hostile - reception of a Ukrainian Greek-Catholic priest by a Latin hierarch; and,
*]20 years earlier, another Latin Catholic hierarch precipitated the largest schism endured by the Catholic Church since the break between East and West by his even more outrageously un-Christian behavior toward a Slovak Rusyn Greek-Catholic presbyter[/list]
Instead, the two pieces dealing principally with the Eastern Churches were authored by Father Adrian Fortescue, an English Catholic priest and liturgist who, despite a deserved reputation as a Byzantine scholar, was an ultramontanist and wrote of the East from that perspective. The third, on Rites, was produced by Patrick J. Griffin, whose credentials to have done so are not immediately available - but whose scholarship as applied to the East appears to have been wanting.
The remaining link, on Eastern Orthodoxy, is an unsigned piece from CAF’s library. On a cursory review, it can hardly claim to be scholarly. laden with opinion, much of which is not encumbered by citations and the accuracy of which as a reliable resource is significantly tainted by this inaccurate statement:
and in 1995, Pope John Paul II and Patriarch Bartholomew I of Constantinople concelebrated the Eucharist together
No one, no one, with even a vague understanding of Catholic-Orthodox relations could read that and not shake their head in disbelief that it appears as a factual statement on the website of a major Catholic apologetic site. Had HH the Ecumenical Patriarch concelebrated with HH the Pope of Rome, the hue and cry raised by the more conservative faithful within both Churches would have reached even to the heavens. Not under the Canons of either Church, Catholic or Orthodox, would such be permissible given the current lack of communio in sacris between the two ecclesia. Yet, that glaring inaccuracy has been brought to the attention of CA staff on at least two prior occasions, some 2 years ago, but has yet to be corrected. On the basis of that alone, putting aside the casual textual presentation, I can’t recommend it as an accurate source from which to learn of the Christian East.
For a brief presentation of the history associated with each of the Catholic Churches of the Eastern and Oriental Communions (and the corresponding Orthodox Churches) from an admittedly Catholic but relatively non-polemical view, I don’t think that one can do much better than Father Ronald Roberson’s The Eastern Churches - A Brief Survey (7th ed) on the Catholic Near East Welfare Association website. I feel relatively certain that my brother and friend, Michael, would concur with me in this recommendation.
On-line excerpts from The Orthodox Church: History by Bishop Kallistos Ware, while no substitute for reading the full text, afford those previously unfamiliar with the history of Eastern Christianity an opportunity to gain familiarity with it from the Orthodox perspective, again in a relatively non-polemical presentation, albeit its focus is principally on Eastern Orthodoxy and doesn’t address the pre-Chalcedonian Churches or those of the Oriental Orthodox Communion. Again, I would anticipate that my brother would agree with me.
As to the history of Oriental Orthodoxy, it is difficult to locate, on-line, a concise piece that is encompassing of all the Churches in the Communion. If there’s sufficient interest, I can pull together pieces on its individual Churches. There is excellent and extensive material available on-line as to the pre-Chalcedonian Assyrian Church. Its history and that of the Ancient Church of the East are the same until a half-century ago, when the schism that separated them occurred.