Defeated in World War I, living in a troubled and insecure peace, Germans under the Weimar Republic were a ready audience for fantasy writers who envisioned victorious wars of revenge or  German renewal through wondrous technology.   Largely ignored by the literary establishment, these writers created an immensely popular mass literature, the Zukunftsroman  (futuristic novel), that was a potent ingredient in the simmering stew of resentment, frustrated nationalism, political irrationalism, and economic distress underlying the Nazi rise to power.     In Fantasy and Politics Peter S. Fisher explores the popular culture of the Weimar Republic.  He identifies common motifs and themes in the work  of thirty fantasy novelists, sets them in the context of political events and ideas, and examines their popular influence.  German fantasy novels, he demonstrates,  provide invaluable perspectives on the ideological and psychological roots of the Weimar Republic's highly emotionalized politics, especially some of its uglier racist and Messianic strands.     The Zukunftsroman was primarily the province of  German right-wing ultranationalists,  who were able to tap the emotional wellsprings of Weimar thought and fill the explosive spiritual vacuum at its core.  However, at the opposite end of the political spectrum, socialist visionaries either exhorted their readers to brace themselves for a final battle with capitalism or wrote pacifist warnings of the terrors of modern warfare.   The preference for the world of fantasy over reality, Fisher contends, was paralleled by an inclination to value action over thought and emotion over reason.   The Weimar Zukunftsroman, he concludes, reveals a cultural drift toward irrationalism that manifested itself politically in the call for dictatorship.