The strife was mainly caused by the unlimited ambition of the pope for power. For centuries this sinful strife threw Europe into political disorder and dragged the Church through the mire of darkest crimes.
Hmmm. There certainly have been some bad popes, but it's rather amazing that they can't even conceive of any sort of legitimacy for a pope having a conflict with an emperor. After discussing one (admittedly) very bad pope, they go on to complain that the popes would try to keep bishops from receiving their offices from the hands of laymen. Though politics and religion were certainly mixed up in a complex way, one gets the sense that the author really doesn't think the church has the right to claim any sort of authority over anyone - even those who are a part of the Church leadership.
Two books we're planning to tackle in our club are Pope Fiction by Patrick Madrid and 10 Dates Every Catholic Should Know by Diane Moczar. We read part of a chapter from the latter book as part of our discussion last night - it was quite helpful.
I do want to note that I recommend the latter book with some reservations. Most of it is an historical work and I think it's quite strong in that regard. A thread of it is more theological and some of this is speculative (which she admits) and points in some directions that aren't particularly helpful or pertinent to a study of history (I'm refering to the "chastisement" thread). Part of what concerns me is that, as I understand it, the concept of "chastisement" has to be understood in a certain way to even be theologically plausible since God does not cause evil but allows it to exist. I also don't like the fact that she speculates on future "chastisements" and how much we seem to deserve this in modern times. This to me seems counterproductive to the purposes of studying Catholic history and feeds, however subtly, into "end-times" hype.