Fantasy is the seed and root of every progressive movement. This is self-evident to some degree, since progressives strive for a condition that does not exist and has never existed, except in the fantasies of the most devout.
Some of these fantasies are meant to be inspirational, like the virtuous proletariat marching arm in arm toward socialist utopia, or the ummah living in Islamic tranquility. Most of these fantasies tend to be fantasies of abstraction, like "fairness" or "justice" or "equality." The abstraction takes the world of the progressive and dessicates it until there is nothing left but the fantastic and unobtainable ideal. But a world that is so narrowly focused ceases to be a world. All progressive movements and their associated fantasies inevitably degenerate into a unitary vision, a homogenizing monotony that is at odds not only with human history, but with the human spirit.
The problem with adopting a fantasy as a world view, is that fantasist often becomes the fanatic, believing not only that his fantasy is desirable to himself but necessary for others. Then, whether it is the degenerate religious fantasies of al Qaeda, or the political fantasies of the Khmer Rouge, or the racial fantasies of the National Socialists, human catastrophe follows.
Fantasy is part of every healthy human mind, but the ability to make the key distinction between fantasy and real life is a necessary part of leaving childish things behind.