Both within political theory and International Relations (IR), recent scholarship has reflected on the nature and limits of political realism. In this article, we return to the thermonuclear revolution and the debates it spurred about what was real and possible in global politics. We argue that a strand of oppositional and countercultural thinking during this period, which we refer to as realist radicalism, has significant theoretical and practical relevance for current scholarship on political realism. Indeed, debates during the thermonuclear revolution speak to questions about the nature of realism and whether it is possible to develop a realism that is attuned to progressive or emancipatory ambitions. By focusing mainly on two radical American intellectuals - C. Wright Mills and Lewis Mumford - we show how their responses to the thermonuclear, superpower standoff challenged conventional understanding of realism and utopianism. By harnessing the concept of the imagination, they called into question pre-existing conceptions about politics and reality. The contribution of the article is twofold. First, we argue that realist political theory and IR should pay more attention to thinkers that are not conventionally regarded as canonical but whose writings and politics interrogated the limits and potential of political realism. Second, we demonstrate that the work of such public intellectuals and their calls for cultivating the imagination connect directly to current debates about political realism, including its statist bend and its (purported) conservatism.