For Theresa May, the most worrying part of Donald Trump’s talks with Kim Jong-un came two days before the two men met. The US President had arrived in Singapore early after escaping the G7 summit in Canada, still sore at being upbraided by his European and Canadian counterparts about tariffs. With time on his hands, he took to Twitter to hit back by switching the conversation to defence and one of his favourite bugbears: Nato.

‘Germany pays 1 per cent (slowly) of GDP towards Nato, while we pay 4 per cent of a MUCH larger GDP,’ he announced. ‘The US pays close to the entire cost of Nato-protecting many of these same countries that rip us off on Trade (they pay only a fraction of the cost-and laugh!)’. This situation, he said, would soon end. ‘Change is coming!’

This was, for once, an understatement. The future of the North Atlantic Treaty Organisation looks shakier now than perhaps at any time since its inception in 1949. Six of the G7 are in Nato, and the US spends more on defence than its 28 other members together. But the alliance only works when its members can present a united front: hard to do when the US President routinely describes Nato as a scam and openly muses about leaving its members to fend for themselves. Without faith in Nato, there is no Nato.

This is precisely why there is such concern in London. Before the G7 summit, Theresa May briefed her ministerial colleagues about the seriousness of the situation — and how the row over tariffs might turn into a full-blown security crisis. The G7 is supposed to be a forum through which Britain, America and Europe deal with China, she said, but this no longer worked because Trump thinks he can handle China all by himself.

Before Trump was elected he had described Nato as ‘obsolete’, but Mrs May told her ministers he only backtracked on this because of pressure from the British government during their meeting in Washington in January last year. The big concern now is the Nato summit in Brussels next month, and that what began with Trump slapping tariffs on European and Canadian steel could turn into a trade war, which the President might then escalate by threatening to walk away from his allies. For Trump, defence and trade are very much linked: his tariffs were introduced on grounds of national security and the need to rely less on imports from supposed allies.

Ivo Daalder, a former US ambassador to Nato, has been frank about what Trump’s conflation of trade and defence means for Nato. ‘The lifeblood of the alliance is trust,’ he said recently. ‘So a trade dispute based on national security — that’s what hurts.’

But it also hurts because there is so much truth in Trump’s criticism. Each of the 29 Nato members agrees to spend 2 per cent of its economic output on defence, but only four do: Greece, Estonia, the UK and the US. Germany, the richest country in Europe, spends just 1.2 per cent of its GDP on defence. Angela Merkel’s offer to raise this to 1.5 per cent is still seen by Trump (and her own defence officials) as insultingly low. Together, European governments are saving about £140 billion by skimping on defence. They do so in the knowledge that the hole will be filled by Uncle Sam.

It’s not just Trump who objects to this. Successive American presidents have been losing patience. Even Barack Obama would complain about ‘the Europeans and the Arab states holding our coats while we did all the fighting’. Britain only agreed to observe the 2 per cent defence spending minimum because Obama had told David Cameron that, without the money, he could forget about a ‘special relationship’. And keeping UK defence spending at this bare minimum has left the British army with fewer soldiers than at any time since the Napoleonic wars. Still, Britain at least has a functioning military. It’s not clear that the same can be said of other Nato allies.

Take Germany, the target of so much of Trump’s recent ire. While defence is a low priority among a largely pacifist German public, mindful of their country’s history, a report for the German parliament earlier this year revealed the extent of the decay of its army, navy and air force. At the end of last year, the Bundeswehr had 128 Eurofighters, of which 39 could fly. It had six submarines, none of which were working when the report was compiled. Of its 13 ageing frigates, only five could sail. Of its 93 Tornado jets, 26 were ready for action. German air force trainees struggled to qualify because so few aircraft were ready for use.

Its staffing is also in crisis. Hans-Peter Bartels, armed forces commissioner to the German parliament, reported recently that 21,000 officer posts are vacant. ‘We spent 25 years cutting the defence budget,’ he said. ‘We thought everything could be solved through negotiations, agreements, co--operation and partnerships.’

Wolfgang Ischinger, a former German envoy to Washington, said it was ‘undignified’ that the most that Germany could contribute to the US-led fight against Islamic State was reconnaissance flights. ‘We take photos, but we leave the dirty business of shooting to others,’ he said. ‘We should not develop the reputation of being one of the world’s greatest freeloaders.’

The reputation of the Germany military has not quite recovered from such fiascos as when, four years ago, soldiers in the- Panzergrenadierbataillon went on an exercise with painted broomsticks instead of guns because of a shortage of weapons. They were part of Nato’s ‘Very High Readiness Joint Task Force’.

Trump’s language about Nato may be vulgar and intemperate, but US officials have been running out of ways to express their frustration. Robert Gates, appointed defence secretary by George W. Bush, warned that Nato would be consigned to ‘military irrelevance’ unless European member states stumped up. Obama seethed about ‘free riders’. When James Mattis was appointed as Trump’s Defense Secretary, he lost no time in warning European leaders that ‘Americans cannot care more for your children’s security than you do.’

So Trump’s tweets are just more succinct expressions of a message that America has been trying to send to Nato for years: patience in Washington has worn thin and it’s time to pay up. If you don’t, then you’ll end up fending off Russians with your broomsticks, pretend bombs and broken helicopters.

America’s focus was already shifting away from Europe and towards China. Trump tends to see foreign affairs mainly in terms with his relationship with Xi Jinping, and he is quite open about the low value he places on the alliances he inherited. ‘I believe in relationships,’ he said a few months after taking office. ‘And I believe in partnerships. But alliances have not always worked out very well for us.’

His theatrical style of leadership, on full display in Singapore, is anathema to Euro-peans — it also underlines the essential difference in worldviews now driving Nato apart. His decision to pull out of the Iran deal was made in defiance of the pleas and protests of his allies, including the UK. Trump believes in nation states, strong leaders and chemistry between leaders. He loathes the idea of committees and consensus. Especially when, as he believes, it leads to America being stiffed.

Officially, Britain says it isn’t worried about Nato — given America’s longstanding commitment. Privately, ministers are in a panic and running out of options. Gavin Williamson, the new Defence Secretary, has warned ministers that Trump’s annoyance with Germany may soon spread to other countries and that Britain might not be exempt from the President’s ire. As The Spectator revealed last month, May is planning a massive cash boost for the NHS to mark its 70th birthday next month. That implies less money for defence and a military kept on the bare minimum, with declining capabilities, when threats are increasing both on land and in cyberspace. Williamson was frank with Mrs May: if she wants to protect the special relationship, extra defence spending is the only language that Trump understands.

Philip Hammond, the Chancellor, dislikes expensive solutions to any problems and is proposing that Britain instead applies pressure on Germany. His latest idea is for Britain to suggest that Germany assuages Trump on the trade front: by making more BMWs in South Carolina. Trump, he believes, needs to be offered an industry-related ‘deal’ in return for at least pretending to believe in the G7, Nato, and the other institutions through which Britain and Europe try to wield power and influence.

But after Singapore, the President will be harder than ever to bring to heel. Whether the summit with Kim will be remembered as a breakthrough or a stunt, he believes that his style of diplomacy has been vindicated and that it’s time to see what else he can do. If he wants to scare European nations into spending more on defence by threatening to leave them to Putin’s mercy, then he’ll probably try that too — to see how they respond.

When Trump last visited the Nato headquarters in Brussels, to unveil a monument to the 9/11 victims, he took the opportunity to tear into countries who don’t ‘meet their financial obligation’ to Nato in a way that is ‘not fair to the people or taxpayers of the United States’. His speech did not include an explicit endorsement of Article 5 of the Nato treaty, a declaration that at attack on one member state is an attack on them all. This was supposed to have been the point of his speech. The White House said afterwards that it should go without saying, but diplomats were left with the impression that the omission was deliberate.

The threat is not that America will choose isolation, as it did in the 1930s, or that Russia will send tanks into Europe. If Vladimir Putin did chance his luck in Estonia, or send submarines on cable-cutting sabotage operations on the Atlantic seabed, no one doubts that the US would lead the response: it still has a military designed to fight wars on two fronts at any one time. But the risk is that Nato will lose its remaining capacity to deter as a united group, making lower-level Russian adventurism more likely. When Putin spots weakness and division, he likes to prod, test and watch the reaction — as Ukraine has found out.

Europe has for years bet that this is a risk that America would not run: that it might complain about Nato, but would not do anything to undermine it. But Donald Trump’s election has changed a great many things. And now, the unmovable force of Europe’s refusal to invest properly in defence is meeting the unstoppable force of Donald Trump on a mission. The next few months will show us whether Nato survives the collision.