This article examines how cognitive and normative ideas influence the ability of the European Union (EU) to formulate common policies in response to international crises such as the 2002–2003 Iraq crisis and the Iranian nuclear crisis (since 2002). It argues that in crisis situations, i.e. in highly uncertain circumstances, ideas often become the principal guide for policy-makers. More specifically, ideas foster interpretations of a crisis along several core themes: above all, how the crisis issue is perceived, which means are deemed to be legitimate and/or effective and, depending on the particular crisis, how other relevant themes are seen, e.g. the appropriate relationship with the United States. Thus, the formulation of common EU crisis response depends on the convergence of these interpretations in Member States – as in the Iran crisis. On the contrary, if Member States’ interpretations diverge beyond a common ‘ideational space’ – as in the case of Iraq – dissonance will be the probable outcome.