It is hard to keep a sense of perspective in the final days of a campaign, amid the cacophony of closing arguments. The background scene is blurred in the sprint to the finish line. Tension and uncertainty have been amplified by a savage terrorist attack. It is impossible to know exactly how the monstrosity perpetrated around London Bridge on Saturday will weigh upon the decisions voters make in the polling booth. But it is fair to suppose that the timing was not coincidental. Whoever plotted that attack, and the one in Manchester two weeks earlier, surely intended to disrupt the democratic process.

There is evidence from radical jihadi doctrines published online that spilling blood over the electoral timetable is a deliberate terrorist strategy. The idea is that horrific acts will provoke a backlash, expressed in support for far-right parties and reprisals beyond the ballot box: attacks on mosques, racist graffiti, assaults on Muslims. This accelerates social polarisation and inter-community suspicion. It furnishes material for propaganda campaigns arguing that Islam and western democracy are incompatible, which is a step towards arguing that they are mortal enemies.

Europe is seen as the soft underbelly the west – where the tinder of tension is driest and may finally catch

It is also apparent from sermons and handbooks that circulate in Isis-inspired networks that Europe is seen as the soft underbelly of western power – the place where the tinder of tension in segregated communities is driest, and where, it is imagined, apocalyptic conflagration may finally catch. Manchester and London were attacked as part of a continuous cultural space that encompasses Paris, Nice, Berlin, Brussels and other cities that have been targeted.

Different European countries have their own domestic policies on integration, on state-mandated secularism, on incarcerating terrorist suspects, and policing the ambiguous boundary between radical religious doctrines and ideological incitements to murder. But those national distinctions are immaterial to the terrorists. Wherever there is a pool of young Muslim men nurturing a furious sense of grievance, incubated by some combination of social and economic marginalisation, there are recruits.

No country is spared on the basis of its more or less enlightened approach to community relations. No European government has a reliable method for neutralising the threat. Also, no method can succeed without collaboration at a European level. Theresa May knew this when she was home secretary. It is why she supported retention of the European arrest warrant in 2014, despite demands by Tory MPs that Britain exercise its right to opt out of the scheme. Security coordination is one of the reasons May cited for supporting the remain campaign two years later. EU membership made Britain “more secure from crime and terrorism”, she said.

She has changed her mind, or decided that equivalent levels of safety are available from a Brexit deal. It isn’t in the interests of either side to withhold intelligence or policing cooperation to gain some tactical leverage in the negotiations. And yet it is also an area – one of many – where the best possible outcome for the UK is the preservation of arrangements that already exist as a benefit of EU membership. In security policy, a successful Brexit requires navigating a load of hazards caused only by Brexit.

That would be less alarming if Britain could luxuriate in a position of strategic semi-detachment from the continent to which it is bound by history, culture and geography. It would also be viable if the traditional alliance with the US looked diplomatically robust, politically wise and culturally appealing. With Donald Trump in the White House it is none of those things. The world does not need new evidence of Trump’s vindictive and infantile nature. But his Twitter jabs at Sadiq Khan in the hours after the London Bridge attack – rebuking the London mayor for his words urging calm in the face of atrocity – represent more than a faux pas. They indicate a pettiness and reckless malice that true allies do not inflict on each other, especially not at times of civil emergency and national trauma.

Yet this is the man into whose diplomatic embrace the prime minister hastened in January; on whom the invitation to a pay state visit in return was peremptorily lavished. The defence of May’s efforts at early ingratiation with Trump is that British interests are always served by proximity to the US, and that deference to the office of the president overrides qualms about the character of its occupant. The reward for such loyalty is meant to be leverage.

That currency has been routinely overvalued, even with presidents who were susceptible to outside influence. Trump is immune not only to persuasion by foreigners but also insusceptible to requirements of basic decency and international protocol. His petulant abandonment of the Paris climate accord proves that much. The idea, once confidently asserted in Tory circles, that May was slotting Britain into the fast track to a mutually beneficial free-trade deal with Washington, and that this prospect would intimidate Europeans into making a desirable counter-offer, has come to nothing. It was guff.

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May’s fealty to Trump has yielded no material benefits while costing her credibility in the eyes of EU leaders. It contaminates the idea of Brexit, casting it not as the pursuit of a practical compromise, but as ideological alignment with the maverick nationalism that has captured the White House.

There is an idealised conception of Britain’s role in the world as equidistant between the US and continental Europe: a bridge between them. That idea is not beyond salvation, but it is challenged by the combination of an unreliable US president and Brexit. It would be handy never to have to choose between Atlantic partnership and continental integration. And in the face of globalised terrorism, the security of western democracy depends on the two spheres overlapping, interlinking. Instead May has set Britain on course towards a lonely diplomatic limbo.

A special relationship with Trump is not available or desirable. But here is the painful truth of Britain’s predicament – and the grim irony, after seven weeks of campaigning: on the eve of an election called with the sole purpose of ratifying and accelerating British detachment from the EU, our cultural affinity with the rest of Europe and our interest in nurturing European friendships have never looked more compelling.