Before leaders of the North Atlantic Treaty Organisation countries convened this week for their latest summit in Brussels, there was much trepidation among the European allies. Their fears were many and varied, but they all centred on the US president, Donald Trump, and what they saw as his possibly wavering commitment to European security.

The Europeans were concerned that his sharp statements about low levels of European defence spending might presage a loosening – or even the end – of the Nato alliance, if the US demanded contributions they felt unable or unwilling to pay. A particular worry was that Trump’s specific complaint about Germany’s defence spending might lead him to announce the withdrawal of US forces stationed in that country.

Then they fretted that Trump’s ambivalence (to say the least) about Russia and his imminent bilateral summit with Vladimir Putin might not only sow discord among the European allies but cast doubt on one of the chief reasons for the alliance’s existence.

In the event, as so often, Donald Trump’s advance bluster proved a lot more ominous than his actual bite. Holding a typically unscheduled and off-the-cuff press conference – which effectively stole the thunder of the official Nato close-of-summit press briefing – the US president gave a good impression of trying to set European minds at rest. He was absolutely confident, he said, that all European allies would meet their 2 per cent commitment, and held out the prospect that it would “go higher”.

He expressed no misgivings whatever about the future of the alliance, saying that it had been “a really fantastic two days”, where there had been “great unity and spirit”, and it had all “come together at the end”.

With that characteristic Trumpian ending to a meeting clearly dreaded by Europeans, the president was already striding towards the exit and Air Force One waiting to carry him to Britain. It is probably fair to say that he left an enormous sense of relief in his wake. The Disruptor-in-Chief had apparently been placated with promises of more money and gone peaceably on his way. The existing order prevailed, and Nato lives to fight another day.

However, while the relief of the Europeans might be understandable – relief, too, it should be said, on the part of Nato’s top brass – such a response would be as irresponsible as it is complacent. Just because Donald Trump was not (for the most part) Europe’s choice for president, just because his bombastic words and his mercantile ways rub up so many Europeans the wrong way, does not mean that he is always wrong.

Indeed, the greater risk is the opposite: that the aversion to Trump in so many parts of our continent is allowing Europeans to turn a blind eye to changes they should be preparing for.

Eighteen months into his presidency, the Europeans seem increasingly to believe that they have fended off the sentiment that initially so spooked them – Trump’s remark during the campaign about Nato being “obsolete”. And, to an extent, that remark was (as politicians like to say) taken out of context. Trump was less consigning all of Nato to history than questioning priorities which, it seemed to him, were stuck in the past and needed to be reorientated towards the more contemporary threats, as he sees it, from terrorism and migration.

Nato leaders gather for summit: in pictures Show all 10 1 /10 Nato leaders gather for summit: in pictures Nato leaders gather for summit: in pictures World leaders gather for Nato family photo Getty Images Nato leaders gather for summit: in pictures British Prime Minister Theresa May arrives EPA Nato leaders gather for summit: in pictures President Donald Trump talks to British Prime Minister Theresa May EPA Nato leaders gather for summit: in pictures U.S. President Donald Trump, left, talks to British Prime Minister Theresa May during a family photo at a summit of heads of state and government at NATO headquarters in Brussels Wednesday, July 11, 2018. NATO leaders gather in Brussels for a two-day summit to discuss Russia, Iraq and their mission in Afghanistan. (AP Photo/Markus Schreiber) Markus Schreiber AP Nato leaders gather for summit: in pictures US President Donald Trump and Nato Secretary General Jens Stoltenberg attend a bilateral breakfast ahead of the Nato Summit in Brussels Reuters Nato leaders gather for summit: in pictures President Donald Trump, right, gestures as he speaks to the media prior to his bilateral breakfast with Nato Secretary General Jens Stoltenberg, left AP Nato leaders gather for summit: in pictures President Donald Trump, US Secretary of Defence James Mattis and US Secretary of State Mike Pompeo at the bilateral breakfast with Nato Secretary General Jens Stoltenberg REUTERS Nato leaders gather for summit: in pictures President Donald Trump and Mike Pompeo, left, are reflected in a mirror as they attend at a breakfast meeting AFP/Getty Images Nato leaders gather for summit: in pictures President Donald Trump speaks during the bilateral breakfast EPA Nato leaders gather for summit: in pictures President Donald Trump gestures during the bilateral breakfast EPA

But the difficulty for Europeans – and for American Atlanticists, too – is that the US interest in underwriting Europe’s security may genuinely be waning. And this has two implications. The first is that the European members of Nato will either have to spend a lot more on defence or organise themselves a lot better, or both. The second is that they will have to look to a time when US involvement in Europe’s defence will be significantly less or nonexistent.

And the point is that, while Trump may have expressed himself more forthrightly than previous US leaders, and thanks to social media reached a wider audience, he is only the latest in a line of American presidents who have called on the Europeans to contribute much more to their own security, and indicated that, with the growing strength of China, the US has to look to its western flank at least as much as to the east.

Both George W Bush and Barack Obama expressed frustration with what their officials referred to as European “freeloading”. In 2011, Obama’s outgoing defence secretary, Robert Gates, used his farewell trip to Brussels to warn in no uncertain terms that Nato risked sliding into “military irrelevancy” if European allies did not take a greater share of the burden.

The Europeans were fortunate, in a perverse way, that Nato’s enlargement to the east and Russia’s military action in Georgia and Ukraine reminded the US of why Nato had been founded in the first place, and encouraged it, for better or worse, to revisit first principles. In this respect, Nato staved off either decline or reorientation – but change cannot be postponed indefinitely.

Aside from convincing the European allies to up their contributions (though precisely by when and how much is not entirely clear), the prospects for Nato after this week’s summit remain ill-defined. There will be minimalist expansion – Macedonia has been promised accession (in part as an incentive for changing its name and calming relations with Greece) and is set therefore to follow Montenegro into the alliance. Russia doesn’t like this, but a Balkan expansion for Nato is vastly preferable to Russia, as both sides know, than accession for Ukraine and/or Georgia.

Trump was – perhaps surprisingly – entirely “on message” when he told his press conference at Nato that there might be a chance of membership for them, but made no definitely commitment whatsoever. How Trump regards Nato in future will greatly depend on what happens at his summit with Putin. If they get on, Nato will slip even further down the US president’s priorities; if they do not, he will probably stick with it (and the European allies) for the time being.

Other factors, though – which include growing US concerns about China, a strong isolationist tendency among US voters which helped bring Trump to office, and the passing of the generations that remember US Second World War solidarity with Europe – all militate against the long-term survival of Nato. If relations with Russia improve – under this US president or the next – it is not impossible that Washington will wind down the defence alliance with Europe, or simply allow it to wither.

The best that Europe could then expect, in terms of a transatlantic security umbrella, would be a federated arrangement with equal and largely autonomous US and European components. This is an option the EU could be preparing for, with its post-Brexit plans for an EU military headquarters and an as yet small military contingent of its own.

Supporters of the North Atlantic alliance – in the US and Europe – will fight such a future, arguing that it gives Russia all it ever wanted and endangers the security of the eastern members. The counterargument, however, is that post-Soviet Russia turned into a threat only because the survival and expansion of Nato after the Cold War sowed the seeds of a new standoff. If the prospect of the US disengaging from Europe’s defence were to encourage Europe to consider new security arrangements with Russia, that could be a more effective way of solving the supposed “Russia problem”.