Opinions vary about the origins of the first world war, but there is no doubt that the rise of aggressively chauvinistic nationalism in Britain and across Europe in the latter half of the 19th century, dressed up as respectable flag-waving patriotism, was a key factor.

As Britain and its erstwhile allies and foes commemorate an awful byproduct of that phenomenon on Friday – the 1916 battle of the Somme – the spectre of unthinking, potentially violent nationalism and its ugly sister, hatred of foreigners, is once again stalking Europe.

The Brexit vote has shattered a lot of prior assumptions, and the belief that Europeans will never, in any circumstances, fight each other again is one of them. A future conflict pitting Britain against its neighbours remains unlikely. But it is not quite as unimaginable as it appeared last Thursday.

Once again the British find themselves caught in no man’s land. Will they accept responsibility for their plight? Or if, as seems likely post-Brexit, national prosperity and security decline, jobs are lost, the country becomes more isolated and there is more austerity, will they blame outsiders?

Externalising domestic problems by blaming foreigners is the well-established tactic of political rogues and demagogues throughout history. Ukip’s Nigel Farage gave a glimpse of how it works this week when he scornfully ridiculed MEPs in Brussels simply for being Europeans.

Jingoistic, fact-free newspapers also play their part. In 1914, exaggerated accounts of German atrocities in Belgium helped beat the recruitment drum, luring countless young men to their deaths. Jingoism reached new heights during the 1982 Falklands crisis. Recent weeks have shown that the tradition of irresponsible media incitement and distortion is alive and well.

There is no shortage of potential flashpoints should a beleaguered Britain lash out again. The Falklands aside, if the Spanish used Brexit to try to forcibly assert control over Gibraltar, as they have hinted they might, recent at-sea skirmishes involving Spanish naval vessels could in certain circumstances escalate.

Disputes over migrants trying to enter Britain illegally, over control of North Sea fisheries or, for example, over the status of British-owned property in France and other EU countries (there are echoes here of Henry V and Calais) could all turn nasty in future. Then there is Cyprus, at the centre of an unresolved conflict between Turkey and Greece, where Britain has legal responsibilities and where it still maintains a military base.

Britain would not necessarily be the aggressor if push comes to shove in a future, disunited Europe. The extreme nationalists of the French Front National, the German Alternative für Deutschland and myriad other hardline European groups are all just as capable of demonising foreigners and turning misery at home into ostracism and aggression abroad.

Beset by strikes and a failing economy, France is already close to a nervous breakdown. What happens if Angela Merkel’s pro-migrant stance costs her next year’s German federal election is anybody’s guess. In Spain, amid record youth unemployment and a street-level uprising, there is barely a government at all.

Then there are pre-existing inter-European troublespots, such as Catalonia, Bosnia, Greece, the Baltic states and Ukraine, that could suddenly flare up, sucking in Britain and other European powers as well as Russia. And this is not to forget English-Scottish tensions, which could grow considerably sharper if the SNP’s EU aspirations are thwarted.

In times of trouble, Britain turns to the US. This was true in the Great War, and again in 1939-45. But the Brexit vote to leave the EU was also a V-sign to Washington, which believes its vital strategic and security interests will suffer, and a galling personal rebuff to Barack Obama.

Despite assurances to the contrary, it was also a body blow to Nato, the key European military alliance. And it has threatened the UK-US special relationship, if only because Britain will matter less in the big picture. In short, the American safety net is fraying.

Perhaps historians will say one day that what is happening is all part and parcel of Britain’s post-imperial decline. From running the greatest empire ever known, its destiny is to become a small, over-populated and not very important island in the north Atlantic.

Perhaps, in fact, Britain’s inexorable decline and fall began 100 years ago, in the muddy, futile battlefield of the Somme.