They are saying that Thursday – the 75th anniversary – will be the last of the international D-day commemorations in which the veterans of 1944 participate. For obvious reasons that may well be so. The surviving soldiers who fought their way up the beaches of Normandy are in their 90s now, so it seems poignantly unlikely that more than a handful will return in 2024.

But there is a more political reason why this week could be the start of a less unified approach to marking the liberation of Europe at the end of the war against Hitler’s Germany. The reason is that Donald Trump’s US and Brexit Britain, though both still immeasurably and justifiably proud of the roles their predecessors played in this epic climax of the war in the west, are each in their own way turning their backs upon the European order that the invasion of 6 June 1944 made possible.

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The uneasy relationship between Wednesday’s major commemorations at Portsmouth and those that will take place on Thursday in France illustrates Britain’s change of stance. In recent decades, Normandy has overwhelmingly been the main place to commemorate D-day. But now here is Britain mounting a large parallel event. Fair enough, in one sense. Britain was the base from which the war in the west was won. It is easier for the 90-year-olds to get to Portsmouth than France. But it is also as though Britain is choosing to reassert a closed-off version of its own national wartime myth alongside – and even in opposition to – the previously more established international one.

This marks another stage in both the systemic impact of Brexit and, at the same time, the older, slow decline in the potency of Britain’s own postwar myths – a process that itself had much to do with the Brexit vote. Postwar British politics was rooted in two competing but also mutually dependent myths that arose directly out of events like D-day. One was a Conservative version of Britishness, which celebrated indomitable exceptionalism, military prowess and long-enduring – especially English – traditions of governance and culture. The other was a Labour version, centred on national unity, shared sacrifice and reward, and an exceptionalism that is more British than English. These traditions overlapped at times. Both included a strong sense of British uniqueness, and each has spent much of the past 75 years seeking ways to recreate their respective idealisations of this imagined past.

Facebook Twitter Pinterest ‘Britain is choosing to reassert a closed-off version of its own national wartime myth.’ Macron, May, Prince Charles, the Queen and the Trumps watch a flyover during D-day commemorations. Photograph: Tolga Akmen/AFP/Getty Images

They remain powerful visions: today, Brexit and Corbynism are respective reflections of their enduring allure to many. But, as time has passed, the claim to national greatness has become more remote from the interconnections of the modern world and from people’s life experiences. Politically, both myths have been challenged by internal “modernising” movements. But they have also been challenged by movements and parties that speak to different post-Churchill and post-Attlee visions of Britain – among them pro-Europeans, environmentalists and an array of identitarians.

These evolving fractures ensure that the solemn events this week cannot just be commemorative business as usual. Instead they mark a change. They embody the chronic uncertainty of the times. They cast an unavoidably unforgiving light on the way that the electorates of both the US and Britain have each taken deeply disruptive decisions that threaten the international order of which their governments still imagine themselves to be mainstays.

In one sense this is merely another phase in on ongoing process. As the decades have passed, the weight which these wartime anniversaries have been called upon to bear has always evolved. In the early postwar period, the D-day commemorations in Normandy served a predominantly Gallic narrative as postwar France sought a way to rebuild. By 1964, when Dwight Eisenhower returned to Normandy for the first time since he had commanded the invasion of France to speak movingly about D-day and world peace, a more cold war and US-led narrative predominated. In 1994, in the first post-cold war commemoration, Bill Clinton launched a heroic new myth about the global benevolence of US power, which George W Bush (in spite of Iraq) and Barack Obama echoed in 2004 and 2014.

Now, though, we have reached a sharp turn in the narrative. Donald Trump’s isolationism and indifference to international institutions such as the EU and Nato mean these will be the most uneasy D-day commemorations for years. And so they ought to be. For Trump represents a defiance of the internationalism and shared interests that marked D-day itself, as well as most of the commemorations over the years. Faced with a president who doesn’t care whether he offends others, it is hard for the collective D-day song to remain the same.

Facebook Twitter Pinterest ‘Internationalism and shared interests.’ Royal Marines at Juno beach during the D-day landings. Photograph: Hulton Archive/Getty Images

But the Britain that Theresa May will represent in Normandy on her departure from office is a destabiliser of the postwar order too. Under May, Britain has protested against its commitment to Europe while readying to break the relationship unilaterally. It has talked the language of rules-based governance while spurning the obligations of rule enforcement. And it has talked about the primacy of trade while flirting this week with a US president who may allow US firms to gouge the NHS and blitz environmental and food standards.

It would be idle to pretend that this is not a splintering moment in the way Europe sees its 20th century past. But these fractures can be healed. These retreats can be reversed. To see how this can happen, consider instead the approach of two people from different ends of the 75-year spectrum being marked this week. One is someone whom, in so many other ways, May might have resembled in other political circumstances. The other is a leader who in every way was the antithesis and moral reprimand to everything embodied by Trump.

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Last week, Angela Merkel went to Harvard to deliver the end of academic year commencement address to the university’s graduating students. She did not mention Trump by name but, in her remarkable speech, told the students not to act on impulses but always to pause and think, to always ask themselves whether courses of action were right rather than easy, and to resist the temptation to “describe lies as truth, and truth as lies”. Above all, Merkel said, be “open-minded instead of isolationist”.

And on the very first anniversary of D-day in 1945, with the war barely won, Eisenhower addressed a cheering crowd in London. He told them that “you, and others like you through all the united nations” had won the victory. And he urged that the bonds between nations – “into which scope must be brought Russia, France, China and all the other great countries” – should never be broken. It is one of the perversities of history that today, 75 years after D-day, it is the German chancellor, not the US president or the British prime minister, who stands for the values that Eisenhower proclaimed all those years ago, and for which the soldiers stormed the beaches in 1944.

• Martin Kettle is a Guardian columnist