The international conflict over Iran's nuclear programme continued for more than two decades before a negotiated settlement was finally reached. This thesis will explain why Iran has proved so resistant to compromise by analysing separate but related factors. Firstly, it will examine the historical roots of today's nuclear co-operation and non-proliferation regime. It will argue that this regime has serious weaknesses and that a better alternative had been available but was rejected in the immediate post-war period. Secondly, it will look at the origins of Iran's hostility to America and Israel, which emerged from a separate dispute with Britain, and which was indirectly related to the political decisions that were taken at the end of the World War II. Thirdly, it will analyse the reaction of international community to Iran's nuclear programme and the rights of states to take unilateral countermeasures under the law of state responsibility. Finally, it will examine the negotiations that have been held between 2002 and 2015 which led to the agreement called the JCPOA. The more general aim of the thesis is to suggest that in conflicts between states which involve their core interests, international law plays a largely instrumental role in helping those states to achieve their political objectives.