This article introduces a mode of social action and interaction that has so far been largely overlooked in the U.S.-dominated international relations debate between rational choice and social constructivism that focuses mainly on the differences between instrumental rationality and norm-guided behavior. Drawing on insights from a theoretical debate within the Germanspeaking international relations community, I suggest that actors have a third mode of social action at their disposal: arguing and deliberating about the validity claims inherent in any communicative statement about identities, interests, and the state of the world. Arguing and truth-seeking behavior presuppose that actors no longer hold fixed interests during their communicative interaction but are open to persuasion, challenges, and counterchallenges geared toward reaching a reasoned consensus. The preconditions for argumentative rationality, particularly a “common lifeworld” and the mutual recognition of speakers as equals in a nonhierarchical relationship, are more common in international relations than is usually assumed. Arguing processes are more likely to occur the more actors are uncertain about their interests and even identities, the less they know about the situation in which they find themselves and the underlying “rules of the game,” and the more apparently irreconcilable differences prevent them from reaching an optimal rather than a merely satisfactory solution for a widely perceived problem (“problem solving”). Moreover, arguing is likely to increase the influence of the materially less powerful, be it small states or nonstate actors such as INGOs. I illustrate these claims empirically with two plausibility probes. The first concerns the East–West talks leading to a negotiated settlement of the Cold War in Europe and German unification within NATO. The second case focuses on the implementation of international human rights norms into domestic practices of Third World states.