Why is there no post-Westphalian world polity today, despite the globalism of recent decades? Is the construction of a world polity an impossible utopia? If it is possible, under what conditions, by what processes, and in what necessary social form? Available visions of a world polity form a debate and world polity formation theories offer limited explanations. In response, this study argues the emergence of a world polity is possible, but is an unlikely and fragile outcome in a late modern context. Two contributions are made to support this argument. First, a new world polity formation theory is developed that explains how systems of polities become single polities. A second contribution advances an account of the historically specific transcivilizational and planetary social form a world polity must necessarily attain if it were to be practically constructed in a late modern context.