This book addresses the question as to where international law fits into the making and implementation of foreign policy during an international crisis in which a State is considering and / or may actually use force. Empirical literature on the law-State behaviour relationship during international crises has not been able to answer this question adequately. The limitations of existing empirical literature are identified as stemming from the limitations of existing positivist, realist and functionalist theoretical explanations of the law-State behaviour relationship. These theoretical approaches, which underpin existing empirical literature on international crises, assume that international law matches what is referred to in this book as its 'rule-book' image. This is the notion of international law as a finite set of objective, politically neutral, rules that can be applied so as to distinguish objectively between legal and illegal action.The rule-book image of international law does not match reality, but the assumption that it is true underpins both theoretical literature and references to international law in political rhetoric. The rule-book image and the reality of international law have been reconciled within the theory of International law as Ideology (ILI) as developed by Shirley Scott. This book hypothesises that an ILI perspective offers a better explanation of the law-State behaviour relationship during international crises than rival explanations grounded in positivism, realism or functionalism.Four case studies of State behaviour - of the US, the Soviet Union and the PRC during the Korean War (1950-1953), of the US and UK during the Suez crisis (1956), of the US and the Soviet Union during the Cuban Missile Crisis (1962) and of the US and an alliance of Latin American States during the Dominican Republic crisis (1965) - are used to test the hypothesis. The findings confirm the greater explanatory efficacy of ILI and demonstrate that the significance of international law to foreign policy decision-making during international crises is more than that of deterring the use of force as is assumed by rival theoretical approaches grounded in a rule-book image of international law. International law is shown to serve as a vehicle for inter-State competition during international crises.