At the Council of Ephesus in 433, Cyril obtained the condemnation of his rival Archbishop Nestorius of Constantinople for heresy. The vote was taken before the eastern bishops who supported Nestorius could arrive. When they did arrive they excommunicated Cyril. Both sides then appealed to the imperial government, then run by the eunuch Chrysaphius, who wisely deposed them both. After a campaign of letter writing and bribery, Cyril was allowed to return and the decisions of the synod endorsed. The Nestorian schism had begun, and has still not been resolved to this day.
After the synod, Cyril’s reputation was tarnished. Isidore of Pelusium wrote to him that, while he agreed with Cyril theologically, a lot of people thought that the Alexandrian Archbishop had behaved like a jerk.
At this time Cyril wrote a number of theological treatises justifying his position. Three treatises De recta fide (On the true faith) were addressed to the emperor and his female relatives. Another Apologeticus was directed to emperor justifying his behaviour. None of these have been translated into English as far as I am aware. Probably the need to understand some fairly complicated theology has deterred many.
I have now found a translator with the right area of interest to work on the Apologeticus. This should cost me around $600. The intention would be to sell printed copies, to recover some of the money, and then release the translation into the public domain. If this works, the other three works might also be done.
There is a text in Migne, of course. P. E. Pusey edited it again in the 1870’s. According to Quasten the best text is in Edward Schwartz’ series Acta conciliorum oecumenicorum. This series has been republished by DeGruyter, here. The series has an unusual structure; the series consists of 4 ‘tomes’ each divided into a number of volumes, and each volume is divided into half a dozen parts. Each part is a separate physical volume, and costs around $200. I need tome 1, volume 1, part 3, it seems, so have requested an ILL.
It will be interesting to see how this goes.