Wikipedia on Charles Tilly:
Examining political, social, and technological change in Europe from the Middle Ages to the present, Tilly attempted to explain the the unprecedented success of the nation-state as the dominant polity on Earth. According to his theory, military innovation in pre-modern Europe (especially gunpowder and mass armies) made war extremely expensive. As a result, only states with a sufficient amount of capital and a large population could afford paying for their security and ultimately survive in the hostile environment. Institutions of the modern state (such as taxes) were created to allow war-making.
Another focus of Tilly’s work is the area of contentious politics. In opposition to individualistic, dispositional analyses of contentious politics, his work emphasizes how social groups organize and contest with each other. In his early years, Tilly also studied migration to cities and American urban phenomena.
In the five-minute video below (part of longer interview available in pieces on youtube), Tilly discusses individualism, social action, and cognitive science.