This is the oldest surviving division in Christianity, and Western accounts routinely lead with 1054 and 1517 instead, which puts the sequence six hundred years out.
The Council of Chalcedon in 451 defined that Christ exists in two natures, divine and human, without confusion, change, division or separation, in one person. The churches of Egypt, Armenia, Syria and later Ethiopia would not accept the formula. Their own confession is miaphysite: one incarnate nature of the Word, in which divinity and humanity are united without mixture and without separation. Their opponents called them monophysite, meaning that they collapsed the humanity into the divinity, and they have denied that charge for fifteen centuries.
The interesting possibility, and it is now the mainstream scholarly and ecumenical reading rather than a fringe one, is that the two sides meant much the same thing in different vocabularies. The Greek word physis was doing different work in Alexandria and in Constantinople, and Cyril of Alexandria, whom both sides claim, had used language that could be read either way. A series of agreed statements between Oriental Orthodox and Eastern Orthodox theologians from the 1960s onward concluded that the christological faith of the two families is substantially the same. Formal communion has still not been restored, which is a fair indication of how much of a fifteen-hundred-year separation is doctrine and how much is everything else.
Politics did the rest. The Chalcedonian definition was imperial policy, and refusing it meant refusing Constantinople. When Arab armies took Egypt and Syria in the seventh century, the non-Chalcedonian churches passed out of Byzantine reach permanently.