Since 11 September 2001, the ‘Muslim world’ has become a novel religio-culturally defined civilisational frame of reference around which American foreign policy has been partly reoriented and reorganised. In parallel, the ‘Muslim world’, is increasingly becoming, at this historical juncture, a civilisational social fact in international politics by being progressively embedded in, and enacted onto the world by, American foreign policy discourses, institutions, practices, and processes of self-other recognition. This article theoretically understands and explains the causes and consequences of these changes through an engagement with the emerging post-essentialist civilisational analysis turn in International Relations (IR). In particular, the article furthers a constructivist civilisational politics approach that is theoretically, empirically, and methodologically oriented towards recovering and explaining how actors are interpreting, constructing, and reproducing – in this case through particular American foreign policy changes – an international society where intra- and inter-civilisational relations ‘matter’.