Donald Trump's astonishing behaviour this week is a threat to Nato's very existence and rationale

As he arrives in Britain, en route to Helsinki for his session with Vladimir Putin, President Trump has created a level of confusion unusual even for him

The point about Nato is that America cannot 'leave' it: without the US, Nato is nothing (AFP/Getty)

Never knowingly modest, Donald Trump has not only claimed that his Nato partners have agreed to increase their defence spending but also that Nato’s secretary general, Jens Stoltenberg, had given him the “total credit” for the achievement.

Not all of those present seem to agree with those impressions, which can only be troublesome for the North Atlantic Treaty Organisation, a body that looks forward to its 70th anniversary next year. For if Mr Trump is right about only one thing in geopolitics, it’s this: only five of the 29 member organisations honour their obligation to spend at least 2 per cent of their national income on defence. This shortcoming, as the president correctly points out, leaves the United States responsible for the great majority of the budget, some way in excess of its fair share.

Mr Trump sees the world in simple – some would say simplistic – terms, but his frustration as he contemplates the imbalance of America’s defence relationship with Europe is understandable.

Around the table at Nato HQ were the leaders of some of the largest economies and richest peoples in the world. Where once, in the years immediately after the Second World War, they needed Marshall Aid and were in no position to fund their own defence against an aggressive and expansionist Soviet Union, which had indeed occupied some nations now liberated and members of Nato, they can afford to pay their bills nowadays.

The point about Nato, however, is that America cannot “leave” it, because without the US Nato is nothing. Mr Trump has openly contemplated doing so, though not immediately. This means one of two things: either the president doesn’t mean it but was making extravagant threats to get the Europeans to take their fair share of the military burden; or else he does think that America can very well live with a Europe left undefended and prey to Russian influence, or worse.

That is the threat to Nato’s very existence and rationale: that even if the US had to pay the entire bill for Nato, it is in America’s interests to have Europe free and not harassed by Vladimir Putin. It would be helpful if Mr Trump could say as much, and preferably in public in the presence of Mr Putin.

As he arrives in Britain, en route to Helsinki for his session with Mr Putin, then, President Trump has created a level of confusion unusual even for him. Though his point about sharing the burden of defence is fair, he has risked destabilising Nato and encouraging the Russians to think that America can be peeled away from this foundation of western security. Mr Trump was almost equally vague about the Russian occupation of Crimea.

He said he was “not happy” with the situation, but seemed keen to blame his predecessor, Barack Obama, who was in the White House when Russia invaded. The fear must be that Mr Trump would be duped by Mr Putin into sacrificing Crimea and relaxing sanctions in exchange for some other concession by the Russians.

That would not be in America’s interests, and still less in Europe’s. Mr Trump described Mr Putin as a “competitor”, a curious word that suggests he has not yet got the measure of the Russian leader.

Not for the first time, therefore, the world is left bewildered by Mr Trump’s behaviour, including some astonishing remarks about Germany being a “prisoner” of Russia.

As always with this president, the rational response is to hope for the best but plan for the worst, and that does mean European partners, inside and outside the EU, making appropriate plans for their mutual defence. That too will involve spending more of their own money.