The rise of antagonism between the German and Czech nationalist activists in the mid-19th century has been neither clearly explained nor convincingly dated. Although this is a topic closely linked to the history of nationalism, the state of research has paradoxically been misguided by the nationalist approach adopted by historians analyzing it. The reason is that nationalism was not the cause but just one response to a greater phenomenon. The aim of the article is therefore to clarify the German-Czech relationship in the broader context of European history and the history of international relations using the perspective of geopolitics and security. As it claims, it was not cultural, linguistic, or constitutional issues but the fear of external threats that caused the mutual distrust of political activists that led to hostility and conflicting policies. Under the impact of international events and within the context of their relations to other international actors this process originated in 1839 by the latest. During subsequent years it developed rapidly and became obvious during the 1848 revolutions. The article thus reveals that this year did not represent the beginning but merely another chapter in a process that had begun nearly a decade earlier.