The article addresses the issue of operational code in foreign policy studies, from the point of view of an individual decision-maker. It presents the conceptualisation of operational code as an instrument of foreign policy analysis, its evolution and application in studies of the decision-making process. By identifying the convictions that constitute the operation code of political leaders, including through the implementation of computer software, we could explain the choices and decisions that individuals take in the process of decision-making in foreign policy. The article includes a case study in which operation code has been used as an instrument to establish the system of convictions of UK Prime Minister Tony Blair, which determined his decisions taken with regard to the Iraq War in 2003.