This article narrates the development of a set of ideas and provocative imagery about population growth and movement that has shaped the way people think about world politics. It represented humanity in terms of populations that could and should be controlled to prevent degeneration and preserve civilization. During the late nineteenth and twentieth centuries, this discursive tradition supported a series of political projects that aimed to either exclude those deemed able to subsist on less and reproduce more or regulate reproduction worldwide. Conceiving of the world in terms of populations – rather than nation-states – led people to think of new ways in which it might be divided, unsettling diplomatic alignments and alliances. But it also contributed to critiques of state sovereignty, since population problems were said to affect everyone and require a united response. This intellectual history helps illuminate some of the local and parochial reasons why people began to ‘think globally’.