WE can never truly imagine their tumultuous feelings of hope and dread.

British, American and Canadian troops setting off from the south coast of England 75 years ago today, first endured horrendous seasickness after deploying in rough seas. American survivors recall how their queasiness was compounded by being served “slimy” British mutton on board, a meal which “slid down fast and slid up just as fast”.

On arriving, many of these nauseous souls disembarked into a cacophonous hail of machine gun and artillery fire, wading painfully slowly through the last yards to the exposed sand as their comrades fell by their sides, their blood turning the sea water red.

On Omaha Beach, where the Americans faced fierce resistance, they had to push across a mine-strewn beach and up steep ridges to overcome the German positions. All told, there were 4,414 Allied deaths during what has become known as The Longest Day. There was brutality on both sides on ensuing days, but the invasion marked the beginning of the end of the war in Europe, and the vanquishing of the Nazi regime.

Is that not one of the enduring lessons for Britain of D-Day, and indeed of the Second World War? That it took co-operation with other countries to defeat fascism? Even 75 years ago, it was becoming clear that the United Kingdom was of declining global importance. Was it not clear in the wake of the Allied victory that Britain’s future security would lie in continuing to reach out to other nations, including former enemies, both to prevent future wars and to build shared alliances against external threats?

Not everyone sees it that way, or at the very least, there is disheartening disagreement about whom we should count as our closest friends. Today, there is a tussle going on to propagandise Britain’s victory in the war, a tussle between, on one side, the anti-Europeans of the hard right, wistful for the past and separation from Europe, and on the other, the outward-looking liberal left who see security in close alliances with the like-minded, both in Europe and beyond. Like Brexit, it is a proxy war for the identity and values of modern Britain.

And so we hear the Eurosceptic Conservative MP Mark Francois preposterously likening his attack on the German-born Airbus boss Tim Enders, to his own father’s fight on the D-Day beaches against “German bullies”. Seen through this distorting lens, Allied troops were fighting on the beaches for freedom against tyranny all right, but now that tyranny has transferred from Nazi Germany to the European project in Brussels and its defenders. Boris Johnson drew a parallel between Nazi expansionism and Brussels plans for closer European integration during the referendum campaign and later suggested that close alignment with EU rules would turn Britain into a “vassal state”. It may be the language of an embittered uncle at a wedding, but it has thoroughly infected the Brexit debate.

When the film Dunkirk came out in 2017, Nigel Farage tweeted a picture of himself, trying and failing to look humble, standing in front of the film poster urging “every youngster” to go out and watch it. Perhaps he was hoping, as a man born in the 1960s who didn’t experience the war, that Britain’s younger generation would acquire the same anti-Europeanism that he did brought up in the era of romantic war films.

The Brexiter-in-chief has also intoned regretfully that Britain started losing its empire as a result of the war and that it suffered a confidence crisis lasting until the 1970s – not, as many would see it, that Britain recognised in those years that it must adapt to survive and that giving independence to its former colonies was both morally right and in Britain’s best long-term interests.

Read more: From the archive: How The Herald reported D-Day in 1944

In fact, the Brexit debate has highlighted just how much Britain’s being on the winning side in the Second World War has allowed many on the right to skate over some of the ugly realities of the British empire. Talk of Britain’s lost “greatness” harks back none-too-subtly to the days of empire, a message that resonates most strongly with some older voters. While some British politicians continue to depict modern Germany in the language of the war, ignoring the tortuous process of painful self-examination and reinvention that country has been through in an attempt to come to terms with its past, it is they who cling to a skewed view of history.

What would all those troops disembarking in Normandy in June 1944 make of all this? We can only speculate. It would be understandable if some, their views shaped in the forge of war, would indeed have backed Brexit out of an unshakeable distrust of Germany. But the wider lesson is surely that those who experienced the tragedy and privations of that terrible time, both in Britain and other countries around the globe, acting with the clarity of thought that comes after tragedy, set up the United Nations and, a few years later, the European Coal and Steel Community, the forerunner of the European Union. Both were established with the express purpose of promoting shared prosperity and preventing war, French foreign minister Robert Schumann describing the aim of the ECSC as being to “make war not only unthinkable but materially impossible”. The United Nations continues to face challenges to its reach and authority, though its achievements are far more impressive than many acknowledge (eradicating smallpox, saving the ozone layer, saving the lives of tens of millions of children and helping prevent, and end, conflicts around the world). The European Union, however, has been even more successful, creating a level-playing field of standards and trade rules across all its member states, setting the scene for national economies to flourish. Above all, it has succeeded in its aim of preventing further war in western Europe and has allowed the countries of Europe to work together in the face of shared threats.

Those who fought on the beaches in June 1944 inhabited a different world, a world in which Britain still had an empire on which the sun never set. But we will be dishonouring their sacrifice and that of the millions of others who died during the war if we cannot have an honest debate about the lessons of the war.