The UK is in the perverse situation of having no coherent foreign policy on the eve of completing an international negotiation that has consumed all of the government’s energy. The situation is less paradoxical than it sounds, because Brexit is a project based on an assertion of what the country does not want to be – a member of the EU – without a clear articulation of what it should be instead. That omission will be felt keenly, but not discussed, at Sunday’s extraordinary summit, where Theresa May and the 27 other European heads of government are expected to agree the broad outlines of a future partnership. But this “deal” will not answer the existential question that Brexit raises: if Britain’s influence in the world is no longer to be deployed as part of a European project, how will it be felt?

The leave campaign did not recognise that as a legitimate question because Eurosceptics always saw “Brussels” as a hostile force and a drain on the sovereign power of a nation state. Brexit, in that view, offered enhanced status as “Global Britain”. That fantasy, like so many of the leaver pledges, has been dismantled by events. The dynamics of a 27-to-one negotiation have demonstrated how a continental bloc asserts power greater than the sum of its parts and greater than any one European country can wield alone. Power in the world is not simply a matter of autonomy to sign trade deals. EU membership has been an axiom of British foreign and security policy well beyond the arena of economic cooperation. To jettison that arrangement would have been risky at any point in recent decades. With Donald Trump in the White House, the timing is exquisitely poor. Mr Trump has started a trade war with China that looks like the prologue to a long confrontation between superpowers. Traditionally, a US president would look to Europe for strategic alliance under such circumstances, but Mr Trump has also picked trade fights with Brussels and heaped scorn on the EU.

Britain does not have to pick a side in this volatile environment but it will have to express priorities. The national interest will often demand alignment with our continental neighbours. That is a fact of culture, history, geography and realpolitik. Historians will look back and wonder how it was that Britain could make such consequential decisions about its role in the world without them featuring in domestic debate. The parochialism of cabinet rows has crowded out strategic challenges. Urgent, global issues are scarcely discussed: climate change; the potential uses and abuses of artificial intelligence and genomics; large-scale population migrations. These are forces that will reshape human society, and it is hard to see how a medium-sized, albeit rich, nation can grapple with them alone.

The most influential actors will be those that can operate on continental scale – China, the US, India, the EU. Such gigantism requires political deftness and skill to navigate. There will be questions about how much freedom ought to be devolved within such superpowers. Once a key player in such debates, we will be a bystander. Britain is recusing itself from that level of influence, downgrading its clout in the world. That might be what many voters want, or at least a price they are willing to pay for Brexit. It is difficult to know, when the question has never been put in those terms. This weekend’s meeting in Brussels is all about Britain and what Mrs May has negotiated. There needs to be more focus on the question of what future British prime ministers will be able to achieve after Mrs May has surrendered the UK’s seat at the summit table.