Why did the Western and Eastern churches react to their respective states so differently?

In *Church History in Plain Language*, Bruce Shelley points out a key distinction between Western and Eastern Christianity: how each has related to the State through history. > Ambrose had hit upon the weapon—the threat of excommunication—which the Western church would soon use again and again to humble princes. But at the center of the Christian empire, in Constantinople, no bishop ever stepped so far out of line. ((https://books.google.com/books?id=RbfVAQAAQBAJ&pg=PA104)) Specific examples will hopefully make the distinction clear. First, in the East, when the emperor [Leo III](https://en.wikipedia.org/wiki/Leo_III_the_Isaurian) forbade the veneration of icons, the [Patriarch of Constantinople](https://en.wikipedia.org/wiki/Germanus_I_of_Constantinople) resigned: > Leo secured the retirement of the Patriarch of Constantinople and the consecration of a new one who favored his own views. ((https://books.google.com/books?id=RbfVAQAAQBAJ&pg=PA157)) But in the West, the bishops fought back. Ambrose excommunicated [Theodosius I](https://en.wikipedia.org/wiki/Theodosius_I) over a massacre he ordered, while Pope Innocent III was especially prolific in applying excommunication and the [interdict](https://en.wikipedia.org/wiki/Interdict) (prohibition of rites throughout a geographic area): > The pope's first weapon in bringing peasants and princes to their knees was the threat of excommunication. [...] Pope Innocent III successfully applied or threatened the interdict eighty-five times against uncooperative princes. ((https://books.google.com/books?id=RbfVAQAAQBAJ&pg=PA194)) This is surely a huge topic, so let me specify: **What is an *overview* of the *theological* differences that led to distinct approaches to unwanted State innovation and overreach in the West and East?** That is: - I'm specifically interested in the use, threats to use, and lack of use of excommunication, the interdict, and similar methods of ecclesiastical discipline, against heads of state and/or their territories. - I'm not interested in historical reasons for the difference. If historical reasons fully explain it (which I highly doubt), then an expansion on "there were no theological differences" would be a good answer. - I'm basically talking about the Middle Ages here: from Ambrose to the fall of the [Byzantine Empire](https://en.wikipedia.org/wiki/Byzantine_Empire) in 1453. Earlier writers probably had an influence, so discussion of them is fine, but I'm not interested in following this into the Modern era. - I'm speculating that Eastern Christians reacted in writing to the excommunications performed by Ambrose and Innocent (and others), while Western Christians reacted in writing to the Patriarch of Constantinople's resignation. If so, their criticisms might shed light on the subject. - I'm interested in an **overview**: a few key points, with a few (sourced) sentences each, would be sufficiently lengthy.