All relationships have boundaries. Those between nations can be particularly fraught, freighted with ties forged in history and culture. In diplomacy the manners, customs and morals of others need to be acknowledged and respected. But humanity begins with acts, not just with thoughts. The question is how to deal with a man like Donald Trump, a taunting braggart with a weakness for flattery? The stakes are high: when nations fall out, people get hurt. By using social media as a flame-thrower, Mr Trump uses words as weapons. He does not care who gets burned.

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In retweeting anti-Muslim video clips promoted by a leader of a far-right fringe group in Britain and then rounding on the prime minister for reproaching him, Mr Trump proves again that he panders to bigots and is no friend of this country. This is an important – and dangerous – moment for Britain as it launches itself into the choppy waters of Brexit. The vain hope of politicians who pushed for this nation’s exit from the European Union was that we could hitch ourselves to the United States.

True, the US is Britain’s most important partner on the global stage. As nations we have a sense of shared values and a long history together. Both have worked to uphold the international rules-based system. After the end of the cold war it was a partnership, along with others, that guaranteed a short period of relative peace. What was not taken sufficiently into account was that this was not only a physical equilibrium but also a moral one.

Mr Trump has few morals. He is a thuggish narcissist who is no respecter of Britain’s national security and wellbeing. After the London Bridge attack in June, he went after the capital’s mayor, Sadiq Khan, for urging, quite reasonably, calm. He attacked Scotland Yard, in September, for not being “proactive” after a terrorist bomb failed to detonate in London. Then, as now, Theresa May rebuked the US president. It was the right thing to do. The prime minister should go further and withdraw the invitation for a state visit. Bullies never respect sycophants. Britain should not allow Mr Trump’s racism to be dressed up in pageantry. Mr Trump’s strategy is to stoke a climate of paranoia, both at home and abroad. He seeks advantage in the politics of division and hate. He operates by instinct rather than sober analysis.

The truth is that Mr Trump has no respect for the basic rights that are the foundation of democracy. Nor does he care for the decency necessary to sustain citizenship. Democracy cannot survive without letting reasonable debate bring the truth to light. Instead Mr Trump appears to have nothing but contempt for our intelligence. For the US president the show is all about one man. His secretary of state, Rex Tillerson, looks set to be replaced by a cheerleader for Trumpism. Mr Tillerson’s error was to realise what everyone suspects: his boss was, in his own reported words, a “moron”.

As a former British prime minister wisely noted, “nations have no permanent friends or allies, they only have permanent interests”. Britain must have a relationship with the United States, just as we have relationships with unsavoury regimes which are tempered by the understanding that we do not share their scruples. Our own foolishness means that we are no longer useful as a bridge to Europe.

The longer Mr Trump is in office, the more America’s folkways will become unfamiliar to Britain. Like all relationships, Britain and America’s will experience rocky times. We are living through one of them. With Mr Trump in the White House the US has become flighty when it comes to “special” relationships, heaping praise on America’s adversaries and downgrading ties with allies. To be credible our bond needs to be grounded in self-respect. Speaking the truth may be difficult, but that is what friends are for.