From Paul Rabinow’s French Modern (1989) to Timothy Mitchell’s Rule of Experts (2002), a rich vein of scholarship has explored the extent to which North African and Middle Eastern territories, societies, and polities were the testing grounds and showcases for European ideas about modernity, progress, and rationality, ideas which found expression in the Arab world while simultaneously denying Arab societies any possibility of autonomous agency in the production of modernity itself. Intellectual histories of colonial discourse and political histories of the colonial state have shown the transformative effects that colonial forms of knowledge and institutions of imperial rule could have in areas from urban space and the economy, to gender and culture. Building on but also departing from some of the themes of this body of work, recent writing on colonialism and empire has looked more critically at the limits of colonial power and the extent to which imperial rule (often a blunt, “arterial” rather than subtly Foucauldian, “capillary” form of power, to adopt Frederick Cooper’s expression) was frequently unable to impose itself except at occasional moments of—often spectacularly violent—intervention. This paper takes up this theme for the post-1940 French imperial state in North Africa, and particularly in Algeria, where the war years and post-war reinvention and reassertion of empire set the stage for political and strategic calculations that would prove disastrously mistaken as the crises of decolonisation escalated. Looking in particular at the private papers of Robert Montagne (1893-1954), the paper asks how a leading intellectual close to the centres of the late colonial state organised information and produced ideas about Maghribi society and politics, ideas that frequently informed late-colonial policymaking. Montagne, a well-known scholar and Maghribi affairs expert, author of the influential Les berbères et le makhzen dans le sud du Maroc (published by L’année sociologique in 1930), was also successively a naval officer, aviator, administrator, educator, political counselor, founder of the IFEAD (today IFPO) in Damascus, and of the CHEAM (Centre des hautes études d’administration musulmane) in Paris, from 1948 a professor at the Collège de France, and throughout his career an assiduous and well-connected student of Maghribi social change. This paper takes his case to show how strikingly the developments of modernity in Maghribi society could be missed and misjudged by prominent colonial experts, whose own forms of knowledge were much more fragile, and much less powerful, than has sometimes been supposed.