State visits are normally a formal representation of harmony between countries achieved through diplomacy. Occasions in which both parties carefully avoid any gesture or allusion that could mar that air of cordiality, or at least good terms. The host puts special care into this, and, of course, so does the guest. It does not occur to anyone to insult someone offering them hospitality. However, it does not seem like the president of the United States has the same concept of what a state visit is, nor how a visitor should behave with those who receive him.

Even before beginning his official visit to the U.K. yesterday, Donald Trump took it upon himself to tell the British government how it should confront the Brexit strategy. The president harshly criticized British Prime Minister Theresa May for walking away from negotiations, supported Euroskeptic Boris Johnson as May’s successor, and added that London should incorporate Europhobe Nigel Farage into talks with Brussels, thereby opting for a drastic Brexit. It is not necessary to recount the grave political crisis that the U.K. is living through due to the failure of the process, which has even led to May’s own resignation. Not to mention the uncertainty in which European countries’ principal economic and political plan puts them. All of them - the United Kingdom and the members of the EU - strategic allies of the U.S. It seems unimaginable that, for example, hours before being received with all honors at the White House, May would reject the way in which Trump is leading the trade war with China. In fact, something like that happening would be unimaginable with any other U.S. president until the arrival of the New York millionaire-in-chief.

But if the criticisms toward his hosts about Brexit go beyond discourtesy, the insults uttered against the mayor of London, Sadiq Khan, moments before landing on British soil are intolerable and shred the dignity that someone who represents the presidency of the most powerful democracy on the planet should remember. Trump, through social media, described the mayor of the British capital as a “stone-cold loser.” It is true that Khan has harshly criticized the visit, but it is the U.S. president who has managed to stoop to a new low. Diplomatic relations cannot turn into a Twitter feud. The alliance between London and Washington is crucial, not only for both countries, but for the democratic world.

And that importance is on display in official visits, something that Trump would do well to remember.