Suddenly the world is full of leaders from Theresa May to President Erdogan of Turkey claiming to unite their countries while visibly deepening their divisions. Denunciations of supposed threats at home and abroad are a common feature of this new political style, whether they are tweeted from the White House or spoken at the podium outside 10 Downing Street.

“Threats against Britain have been issued by European politicians and officials,” said May this week, accusing them of deliberately trying to influence the results of the general election on 8 June. All this sounded very like Hillary Clinton convinced that Russia helped lose her the presidential election, though in the case of Britain any such calculation is highly unlikely given the common European assumption that Mrs May is going to win a landslide victory.

Defending the motherland against the evil schemes of foreigners is a political gambit that has been played out countless times since the age of Pericles, but its impact depends on the political context in which it is used. At the moment, it is peculiarly destructive as ethnic nationalism reasserts itself as a vehicle for grievances and rivalry between different nation states is reaching new heights. Populist nationalist leaders from Manilla to Warsaw to Washington are promising more than they can deliver and looking for scapegoats at home and abroad to blame when things go wrong. Nationalism has always needed real or invented threats to super-charge communal solidarity.

In an age of reinvigorated nation states, English nationalism is more dangerous than it looks. It displaces a vaguer and more inclusive British nationalism, dislocating England’s relations with Scotland and Northern Ireland. It may well be that Scotland will not become independent or Northern Ireland unite with the Irish Republic, but these options are already feasible enough to preoccupy the British state.

Theresa May's full statement marking the dissolution of Parliament

One destructive element in English nationalism is seldom identified. People in England understandably resent the way that their nationalism, which they see as merely sticking up for their own interests, is condemned as racist and jingoistic when Scottish and Irish nationalism (or for that matter Algerian and Vietnamese nationalism) are given a free pass as the laudable pursuit of liberty and self-determination.

There is something in this, but there is a difference between the nationalism of weak countries, whose history is one of foreign conquest and occupation, and the nationalism of larger and stronger ones who did the conquering and the occupying. Smaller countries or embattled communities always play with a weaker hand of political cards than their opponents and cannot do what they like, but this has the advantage of giving them a good grasp of the realities of power.

But states like the US, Britain, France and Russia who have an imperial past or present, have a much less accurate sense of what is feasible and what is not. Their nationalism is coloured by self-justifying myths about their own superiority and the inferiority of others. This is not just distasteful but carries the seeds of frustration and defeat. The British empire fatally underestimating the resistance of Afghans and Boers in the 19th century and the Irish, Indians and Greek Cypriots, among others, in the 20th century.

One could see the same self-destructive arrogance at work more recently in Iraq after 2003 and in Afghanistan after 2006. Public opinion at home never took on board the extent of British failure there was more complete than that of the Americans. In Iraq, the British force ended up signing a humiliating agreement with a Shia militia. No lessons were learned from defeat, as witness Boris Johnson’s glib promise to join the US in attacking President Assad.

In pictures: European parliament Brexit discussions Show all 12 1 /12 In pictures: European parliament Brexit discussions In pictures: European parliament Brexit discussions European commission member in charge of Brexit negotiations with Britain, French Michel Barnier listens at the President of the European Commission Jean-Claude Juncker speaking at the European Parliament in Strasbourg Getty Images In pictures: European parliament Brexit discussions Frank-Walter Steinmeier, President of the Federal Republic of Germany, delivers his speech at the European Parliament in Strasbourg EPA In pictures: European parliament Brexit discussions European Union's chief Brexit negotiator Guy Verhofstadt, President of the Group of the Alliance of Liberals and Democrats for Europe (ALDE), addresses the European Parliament during a debate on Brexit priorities and the upcomming talks on the UK's withdrawal from the EU Reuters In pictures: European parliament Brexit discussions Michel Barnier, European Chief Negotiator for Brexit reacts during a meeting at the European Parliament in Strasbourg EPA In pictures: European parliament Brexit discussions Member of the European Parliament and former leader of the anti-EU UK Independence Party (UKIP) Nigel Farage wears socks with Union Jack flag at the European Parliament in Strasbourg Getty Images In pictures: European parliament Brexit discussions Nigel Farage, United Kingdom Independence Party (UKIP) member and MEP, addresses the European Parliament during a debate on Brexit priorities and the upcoming talks on the UK's withdrawal from the EU Reuters In pictures: European parliament Brexit discussions European commission member in charge of Brexit negotiations with Britain, French Michel Barnier gestures during speeches at the European Parliament in Strasbourg Getty In pictures: European parliament Brexit discussions The President of the European Commission Jean-Claude Juncker (L) speaks with European commission member in charge of Brexit negotiations with Britain, French Michel Barnier at the European Parliament in Strasbourg Getty In pictures: European parliament Brexit discussions European Union's chief Brexit negotiator Guy Verhofstadt, President of the Group of the Alliance of Liberals and Democrats for Europe (ALDE), addresses the European Parliament during a debate on Brexit priorities and the upcomming talks on the UK's withdrawal from the EU Reuters In pictures: European parliament Brexit discussions Member of the European Parliament and former leader of the anti-EU UK Independence Party (UKIP) Nigel Farage gestures during speeches at the European Parliament in Strasbourg, eastern France, on April 5, 2017. The European Parliament will on April 5 lay down its "red lines" for negotiations over a Brexit deal, on which the assembly will have the final say in two years' time. / AFP PHOTO / Sebastien BozonSEBASTIEN BOZON/AFP/Getty Images Getty In pictures: European parliament Brexit discussions German president Frank-Walter Steinmeier delivers a speech during a plenary session at the European Parliament in Strasbourg Getty In pictures: European parliament Brexit discussions The European Parliament in Strasbourg, eastern France Getty Images

Much of this sabre-rattling over the last week is simply part of Britain’s long-standing effort since 1940 to demonstrate its continuing usefulness as the main foreign ally of the US. But here again the political landscape is changing in a way not seen since the collapse of the Soviet Union in 1991. US international leadership under Donald Trump is “mercurial and unpredictable” and Britain needs to rethink its policies in the Middle East according to a report by the House of Lord’s international relations select committee this week. Its chairman, the former Conservative cabinet minister Lord Howell, says that “in a world less automatically dominated by the US underpinning security in the region, it is no longer right to have a stance at every stage of “if we just get on with the US everything will be alright”.

This is all very true, but does not answer questions about whether or not Britain, if it does not piggy-back on US military power, has the inclination and resources to play a more independent role.

There are other doubts about how far British power and influence will survive post-Brexit. Not many Leave voters will have truly believed in Shakespearean rhapsodies about England as “a precious stone set in the silver sea”. But the proponents of Brexit were always cavalier about where Britain outside the EU would stand in a world which is getting more unstable. Appeals of varying degrees of sophistication to the spirit of 1940 forget that British victory in the Napoleonic wars and both World Wars depended on the Royal Navy and on building up a network of alliances with other powers. Having spurned the EU, this latter strategy is going to be very difficult to pursue. Already May, Johnson and assorted royals have been scurrying off to see unsavoury allies in Saudi Arabia, Turkey and among the kleptocratic monarchs of the Gulf.

One aspect of British decline is underrated: people favouring or opposing Brexit both speak in the future tense about the benefits or disasters that will ensue as Britain negotiates its departure. But one of the worst consequences of the decision is already with us and is simply that the British Government is wholly focused on Brexit to the exclusion of everything else.

Mrs May’s explanation that she called the general election to strengthen her hand in negotiating with Brussels is an admission of the dominance of the issue. There is not going to be much time to consider new policies for a changing Middle East or for anything else.

How did all this happen? In many respects, globalisation has turned out to be more destructive to the status quo than communism ever was. In its name, nationalism was discarded and derided by ruling elites who had an economically respectable reason to distance themselves from the rest of society and did not see that they were cutting through the branch on which they were sitting. The left never much liked nationalism, suspecting it of being a mask for racism and a diversion from more important social and economic issues. Populist nationalists came to power in country after country as others retreated from nationalism and they filled the vacuum.