In the Habsburg empire from the 1870s, Croatia became the weakest link in the Rechtsstaat. As governor of Croatia, Karoly Khuen-Héderváry politicized the judiciary and weakened the jury system, providing ample scope for state interference in the judicial process. One result was that the regime used treason law prolifically to curb dissent and silence those Croats and Serbs who did not conform. Contrasting this with other parts of the Monarchy, this paper explains the Croatian context and offers case studies of ‘treason’. In 1887 the poet August Harambašić was prosecuted; twenty years later, the regime with dubious evidence staged a major treason trial of Croatian Serbs. The paper shows how this judicial malpractice seriously damaged Croatian social cohesion but also destabilized the Habsburg empire before 1914.