Modem international law was founded on a persuasive argument: Grotius' reasoning that agreements between States were analogous to agreements in private law. The development of international law continues to depend on techniques of argumentation in many contexts -in the work of the Legal Adviser to national governments finding justifications for State action as witnessed in the arguments of States during the tense period leading to military action against Iraq. Legal advisers use a range of arguments, arguments of analogy, sacrifice, continuity and authority as well as pragmatic and consequential arguments. In the negotiation of international treaties States often appeal to consequential arguments, e.g., the constitutional difficulty (or impossibility) of accepting a certain provision and hence the inevitability of their non-participation if the provision is not changed. In the interpretation of treaties many rhetorical skills are called on: to flesh out vague terms or deal with ambiguities, and the use of multiple authentic languages opens up a wide are for the use of persuasive techniques as to which of the many nuances of the several languages involved should be adopted. Furthermore, international law has heavily depended on the development of juristic doctrine by scholars, and their works are full of examples of rhetorical reasoning, as are the judgements of international tribunals. Some examples are drawn from the decisions of the International Court of Justice: the argumentum ad hominem, the argument of the absurd, the argument contrasting the real and the ideal, the argument as to consequenses, the argument a fortiori. Evident also in international judicial opinions is that hallmark of non-stringent reasoning, the awareness and carefull cultivation of the audience. International lawyers, like jurists in other systems of law, depend greatly on the techniques of reasoned persuasion, and the forms of strict logic play a minor part in the justifications of their decisions. The need for flexibility in reasoning techniques has been clearly expressed in a recent decision of the International Court of Justice.