As Britain sees it, a state visit is an important way of celebrating and strengthening the relationship between two sovereign countries. It is a symbolic occasion intended to formalise and promote a voluntary alliance linking two independent peoples. The principal focus is not on individual politicians, except in so far as such leaders are seen as representing their fellow citizens. State visits are certainly not supposed to be an ego trip.

This is something, among many other things, that Donald Trump plainly does not understand. When the Queen hosts a foreign leader, she does so in her capacity as head of state, not as a tour guide or re-election campaign prop. The honour she bestows belongs entirely to the country of which her guest is the current, temporary leader. Whatever he may think, the state visit is emphatically not a tribute to Trump. Ever since Theresa May rashly issued the invitation, he has pushed for the proffered prize with shameless importunity. Evident reluctance and delay in Westminster and Whitehall was by no means wholly the result of Brexit distractions. By his almost daily actions, Trump has demonstrated such disrespect and disdain for Britain and its national interests as to lead many people here to hope that he would not come at all.

It is fair to say most British people have traditionally supported the US-UK alliance in the post-1945 period. But Trump has done much to shake that instinctive amity. America’s global reputation has plummeted since he took office in 2017 and Britain is no exception. According to a Pew global attitudes survey of 25 countries last autumn, 70% of respondents lacked confidence in Trump to do the right thing.

In Britain, the percentage of people holding a favourable view of the US in the Trump era has fallen to 50%. Across western Europe, the figure is even lower. Only 28% of those surveyed believe the US takes the interests of other countries into account, while 37% believe the US is doing less to address major global problems such as the climate crisis. As a leader, Trump is rated below almost everybody, including Russia’s Vladimir Putin.

This is what Britain must deal with over the next three days. It will not be an edifying spectacle.

None of this fazes Trump, who continues brazenly to conflate and confuse his personal and political interests with those of his country, both at home and abroad. This self-serving elision is what underlies, for example, last week’s threat to impose punitive trade tariffs on Mexico. Trump variously claimed this was necessary to curtail illegal migration, stop drug trafficking and enhance security. In truth, it was mostly about personal spite, occasioned by his failure to force Mexico to fund his promised border wall.

This pattern of rampant narcissism trumping national interest and sound policy repeats across the board. When Trump complains about China’s trade practices, debt diplomacy and Huawei, he may have a point. Few would disagree when he says the activities of Iran’s Revolutionary Guards Corps, particularly in Syria, threaten Middle East stability. It’s true that past attempts to induce North Korea to eschew nuclear weapons have failed. But having identified such problems, Trump proceeds to make them worse, cajoling, bullying and threatening, imposing facile quick fixes, then backing down, offering summits, then flouncing off, saying one thing one day and another the next. This is a problem of vision and strategy, both of which he lacks. It is a problem of ignorance and arrogance, both of which he has in spades. Most of all, the problem stems from Trump’s chronic propensity for making it all about him. As far as he can see, he is the US.

This is what Britain must deal with over the next three days. It will not be an edifying spectacle. And it already going wrong. Judging by his comments to the Sun, Trump thinks he has the right to pick Boris Johnson as Britain’s next prime minister. He reckons his endorsement will decisively swing the Conservative leadership contest. And, as with his unhelpful Brexit interventions, he doubtless believes he is acting in Britain’s and America’s best interests, when, in fact, the very opposite is true.

There are good reasons for the convention barring foreign leaders from meddling in the internal politics of friendly countries. To do so is impolite and provocative. It strains the friendship they ostensibly wish to strengthen. And it can be counterproductive (as Trump-dissed Michael Gove may hope is the case now). Trump’s personal preferences, foolish and ill-judged, are not necessarily those of the American people or state. He does them, and us, an egregious disservice by conflating the two.

It may be asking too much, but Trump should mind his mouth before more damage is done. This is especially important given his participation in this week’s D-day 75th anniversary ceremonies. Trump never served in the military. He dodged Vietnam on health grounds. He has absolutely no concept of the selflessness involved in risking one’s life for others. In Portsmouth, as in Normandy, this will be a very solemn moment marking the sacrifices of thousands of brave men and women. Trump may be present – but he’s not included.