Reflections on a Revaged Century by Robert Conquest

Conquest attributes the disasters of the 20th century to the related ideologies of Communism and Naziism. He explains how both movements recruited and created true believers who would commit any crime to gain the ends of the party. And crimes they did commit, the Soviets killed tens of millions of Russians during the terror of the 1930's, the Nazi's killed 6 million Jews, a million or so other Germans, and then started World War II which resulted in more tens of millions of dead. Conquest defines a dangerous ideologue as a person willing to commit crimes against people to further their ends. Clearly some of the blogosphere qualify here.
Conquest declares that George Orwell (1984 & Animal Farm) and Arthur Koestler (Darkness at Noon) did as much, or more, to bring down communism than anything else.
Moving on, Conquest mentions modern fads of environmentalism, global warming, and multi culturalism as insipient ideologies gathering strenght. His book is copyright just be for 9/11 and as a consequence does not mention Islam.
It is a good read.