The significance of ideas to foreign policy analysis remains contested, despite a plethora of empirical studies applying ideational frameworks. Drawing on social constructivism, this article proposes a causal understanding where ideas derived from tradition define the political space for contemporary debates and effect foreign policy behaviour. This ideational approach is substantiated by a historical study of guiding principles in British and French foreign policy, which establish a set of baseline expectations for the analysis of Tony Blair and Jacques Chirac's speeches on Iraq. The empirical study shows that whereas Chirac largely stayed within a French ideational framework, Blair applied a more complex combination of ideas from both traditions. Conceptualising Blair as an aspiring (but ultimately unsuccessful) norm entrepreneur is a fruitful interpretation of this role.