This paper proposes a theory of competitive agglomeration—a new enquiry into the origins of hierarchical structures and governments. As a motivating example we analyze the Viking age—the roughly 300 year period beginning in 800 AD—from the perspective of the economics of conflict. The Viking age is interesting because throughout the time period, the scale of conflict increased—small scale raiding behaviour eventually evolved into large scale clashes between armies. With this observation in mind, we present a theoretical model describing the incentives both the defending population and the invading population had to agglomerate into larger groups to better defend against attacks, and engage in attacks, respectively. We tentatively postulate that competitive agglomeration during the Viking era was a key impetus to state formation in Europe.