Historians have long recognized the importance of the Roman-canonical maxim ‘defense of the realm’ to the propaganda politics practised by later Capetian kings, most notably in the climactic struggle between Philip the Fair and Boniface VIII. Although rightly conceding to Roman law the major impulse to its formulation, they have been less sensitive to the way in which the doctrine fits into historical images of Capetian kingship. The theory of the king as defender of the realm was critical in the early as well as late middle ages. In tracing the evolution of this slogan through the chronicle tradition of Saint-Denis, the most extensive and consistently royalist historical corpus in Capetian France, one sees the emergence of concepts congruent with those developed in legal and canonical texts, but which reveal little or no influence from these more learned sources. This article argues that the chroniclers' persistent focus on the image of the king as royal defender facilitated the interior evolution of the meaning of ‘defense of the realm’ from that of feudal tuitio to the public concept of Roman jurisprudence. The chroniclers of Saint-Denis thus testify to the way in which older theories of monarchy were being subtly transformed by the changing nature of kingship itself. But at the same time they lend an air of coherency and familiarity to one of the most disruptive periods of Capetian history and sustain a sense of the evolution of the French monarchy within the context of fundamental notions governing medieval kingship.