The idea of a perfect national political community, entirely confined within the contours of a corresponding state, is one of the foundational fictions of global modernity. Its formal crystallisation in the legal grammars of the right to self-determination has been however, particularly in the post-colonial era, highly problematic and full of ambiguities. Drawing on this background, this article contends that diplomacy offers frequently a more promising venue for dealing with the challenge of political pluralism than appealing to either the unstable grammars of the right to self-determination or a reified understanding of the principle of territorial integrity of states. In so doing, firstly, the right to self-determination is critically examined. Instead of attempting to articulate its formal content, the malleability of its legal grammars and political realities, will be emphasised. Secondly, based on the discussion of a variety of historical cases, the notion of ‘constituent diplomacies’ will be advanced, aiming to show how the agonistic accommodation of political and territorial pluralism through diplomacy was crucial not only in the formative processes of modern sovereign states but also nowadays. Finally, this relational understanding of the historical forms of governance of political pluralism within and beyond state boundaries will be re-examined, beyond its ethno-political dimensions, through the prism of the complex interplay between the material and ideational conditions for the co-production of sovereignty in the context of new global capitalism.