The biggest problem with studying history is remembering that the people taking part in it didn't know what was going to happen next. And there is another problem as well - they often didn't know accurately what had happened before either. People's motivations are often therefore hard to fathom. And the existence of conspiracy theories makes it even harder. It is in the nature of conspiracy theories that they tend to be very specific to particular times and are often completely forgotten about later. Take for example the Protocols of the Elders of Zion.
I don't think it is safe to assume that a modern reader has even heard of this particular document. I am pretty sure that the only time I have ever come across them was in a reference to them in some journalism by George Orwell. But when he was writing in the Thirties Orwell just assumed that everybody knew what they were and that they were a forgery.
The Protocols purported to be the minutes of a meeting of senior Jews that took place in Constantinople and which laid out their plans to take over the world. The mode of operation was to be buying up newspapers and magazines and infiltrating political parties of all shades of opinion. The network thus created would exert effective control allowing the Jews to run things basically in their own interest.
It isn't hard to see that believing in such a conspiracy might, at the very least, have made some of the tragic events of the middle of the Twentieth Century easier to contrive. Although the Protocols were conclusively shown to be a forgery it is only to easy to imagine them being widely believed. And people like the editor of the Times and Winston Churchill were prepared to consider that they might have some truth to them.
While that one is a bit obscure - although to my mind the most interesting one of the book - most of the conspiracy theories covered are pretty familiar and you'll have met proponents of some of them unless you lead a particularly sheltered life. Aaronovitch does a great job of dissecting them, and does it in a surprisingly sympathetic way. The theorists themselves are treated with a level of respect that doesn't really accord with the demolition of their ideas he also serves up.
This is an interesting perspective on something that doesn't often get covered. Conspiracies are a part of history, but so are the theories about conspiracies that never happened. It is a good question as to which has had the bigger influence.