Jürgen Habermas' thinking gained influence within the German International Relations (IR) community in the early 1990s. At the core of the so-called ‘ZIB-debate’ was the controversy whether rationalist theory can explain interstate cooperation. Constructivists accused rationalists of ignoring communication, language and reason, thereby leaving a logical gap in their analyses of interstate cooperation. This gap exists between the plausible motivation for states to cooperate and their actual achievement of cooperation. Rationalist approaches assume that actors face problematic, interdependent situations, in which they can only optimise their preferences by collaboration. Such situations involve a plausible motivation for actors to cooperate but they are not a sufficient condition to ensure that cooperation will actually happen. What is missing is a theoretical link between a general motivation to cooperate and its realisation, given that rationalist theory, following realism, regularly assumes a dangerous anarchical environment with its daunting security dilemma. These circumstances make cooperation risky and should normally work as a show-stopper for nation-states to collaborate.