The geopolitical landscape is changing fast. Donald Trump seems to be shaping up for an early confrontation with China. President Putin has emerged strengthened, so far, from his Syria gamble (though after the assassination of his ambassador to Turkey, the long-term costs remain to be seen). Once Trump is inaugurated next month, it would be in keeping with his impulsive style to go for a grand bargain with Vladimir Putin. If so, key issues for European security would be on the table. The stakes for Nato in particular will be high.

Trump has complained that many European allies have cut their spending too far and are in effect free-riding. He’s right on this point. Britain is one of the few European Nato states to meet the target of 2% of GDP spent on defence. But Trump’s cavalier approach to the US commitment to Nato and his evident fascination with Putin risk doing grave damage to the alliance that has been the basis of our security since 1949.

The core of Nato is the undertaking that all members give to each other in article 5 of the founding treaty, which states that “an armed attack on one …shall be considered an attack on all”. This deterrent only works if the guarantee is credible: an adversary has to calculate that the US would be willing to use its military might to defend its European allies. For months, Barack Obama has been reinforcing that credibility by sending US armoured brigades to Europe.

But Trump has gone further than any previous American president in threatening to walk away from the article 5 commitment if other allies don’t pay more. In his set-piece foreign policy speech last April, he said: “The countries we are defending must pay the cost of their defence and, if not, the US must be prepared to let these countries defend themselves.”

It has never been the Nato deal that you only get as much solidarity from the US as you pay for

This was reckless. It has never been the Nato deal that you only get as much solidarity from the US as you pay for. And it is precisely the wrong time for any weakening of America’s security commitment to Europe, with Putin’s seizure of Crimea, his fostering of civil war in Ukraine, and increasingly brazen cyber attacks, including in the US election campaign. He’s also been engaging in nuclear blackmail with the deployment of Iskander nuclear-armed missiles to Kaliningrad.

We’ve been here before. I was working for Nato in the late 1970s when Moscow stationed SS-20s within striking range of western Europe. Then, the response was Nato’s dual-track decision of 1979 to send US nuclear missiles to Europe and offer a deal to eliminate all such systems – which, years later, Mikhail Gorbachev eventually accepted. The relevant point for today is that clear US leadership in Nato was necessary to deter Moscow.

Obama has tried to reassure us that Trump doesn’t really mean to undermine Nato. But it’s vital that the president-elect himself confirms he will stand squarely behind the US commitment to its European allies. And that he avoids a deal with Putin which involves, for instance, pulling back on the forward basing of US forces in eastern Europe, or no further Nato enlargement.

These disconcerting shifts in the global balance should pushing European governments to spend more on defence, and more wisely. Defence is one of the areas (fighting terrorism and foreign policy are others) where EU countries know Britain makes a huge contribution to European security. Given the many risks, in the Brexit negotiations I believe the other EU states will see they have a strong interest in continuing close cooperation with Britain. We should signal early on in the talks that we see things the same way. After all, a Europe actively tackling the security threats it faces has to be in our interests as well.

At the same time, we can be intensely relaxed about the prospect of an EU army – for the simple reason it isn’t going to happen. The idea that the French, for instance, would ever put their armed forces under supranational EU control is laughable. It’s been tried before: in the early 1950s the apostles of European integration, Jean Monnet and Robert Schuman, invented a scheme to pool sovereignty over military power. The result was an elaborate plan for a European defence community, fusing the armed forces of the member states, complete with a parliamentary assembly, a council of ministers and a court. The French parliament vetoed it, and the same would happen today.

So Britain’s top priority must be to persuade Donald Trump of the importance of a credible US guarantee to Nato. The security interests of European states are far too important to be traded off in a US deal with Putin.