The Nato treaty was signed in Washington 70 years ago this week, but British fingerprints are all over it. Having been instrumental in the creation of the transatlantic alliance, Britain now needs to be producing the ideas and energy to keep it relevant to threats undreamed of by its founding fathers.

Prominent among those founders was Ernest Bevin, foreign secretary in the Attlee government and one of the great strategists of the post-war years. Like Churchill, Bevin was quick to see that the threat to western Europe would not come from a revival of German militarism, but from an expansionist Soviet Union. The main risk in the late 1940s was that Moscow-supporting communists would come to power through the ballot box, at a time when their support was running at 25% in Italy and 20% in France.

In a magisterial cabinet paper in early 1948, Bevin therefore called for “the creation of some form of union in western Europe … backed by the Americans and the dominions. We in Britain can no longer stand outside Europe and insist that our problems and positions are quite separate from those of our European neighbours.” Wise words, which are still apposite today.

Bevin practised what he preached. He led the way in creating a European collective defence organisation in the shape of the Western Union in 1948. With that in his pocket, he went to Washington to persuade the Truman administration and Congress that the best way to protect US national security interests was not to pull back, as they had done after the first world war, but to underwrite European security by establishing Nato.

The secret of Nato’s longevity is twofold. First, it is not just a military pact but an alliance of shared values. The preamble to the treaty commits members to “safeguard the freedom, common heritage and civilisation of their peoples, founded on the principles of democracy, individual liberty and the rule of law”. Second, it couples a strong political commitment to collective defence with discretion on how to carry it out. If one ally is attacked, others don’t automatically have to go to war. Each member state is required to “take such action as it deems necessary, including the use of armed force”. Nato has always operated on the basis of trust, not legal obligation. These two features have given it the flexibility to adapt.

Facebook Twitter Pinterest ‘British fingerprints are all over it.’ The Nato treaty is signed in Washington DC, 4 April 1949. Photograph: US National Archives and Records Administration/HANDOUT/EPA

For 70 years, Nato solidarity has held firm. In the cold war, the Americans protected European allies with their nuclear weapons, thereby putting their own homeland into the equation of mutually assured destruction. After 1989, the US joined the Europeans (eventually) in stopping ethnic cleansing in the Balkans, and the Europeans and Canadians invoked the mutual defence article 5 of the treaty (for the first and only time) in the hours after the US was attacked on 9/11. All other allies followed the US into Afghanistan, and paid a high price, with more than 1,000 of their military personnel killed.

After two decades of discretionary wars, Nato has now come back home, giving top priority to the security of allies in the face of Russia’s aggressive military posture and reckless behaviour. Soldiers from many Nato nations including Britain and the US are back on duty near the eastern frontiers of the alliance. There is a new push to improve the readiness of alliance forces to deal with threats of all kinds, including the Russian speciality of exploiting the grey area between intimidation and subversion and outright conflict.

So Nato remains highly relevant, and it is welcome that leaders will be assembling for a 70th anniversary summit in London at the end of the year. But at the same time it is a more fragile alliance than it looks, with differences between allies eroding the mutual trust on which it was built. On the US side, exasperation at the failure of many European allies to bear a fair share of the burden of defence spending has been mounting for decades. At the 2014 Nato summit in Wales all member states committed to working towards spending 2% of GDP on defence by 2024. This year only five, including Britain, meet that target, and major countries such as Germany (which spends 1.2%) have made little if any progress towards it. President Trump has sharply stepped up the pressure and openly questioned whether Nato represents value for money for the US.

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This in turn has led European leaders such as the French president, Emmanuel Macron and the German chancellor, Angela Merkel, to talk of EU strategic autonomy and even an EU army, hinting at ambitions for the EU to go it alone that Britain, as a member of the EU, held in check for decades. The reality is that European countries could not begin to substitute for the military assets that the US makes available to defend Nato allies. Yet the rhetoric in European capitals could accelerate the shift of US priorities away from Europe and towards their top national security priority, the growing confrontation with China.

A good objective for the London summit would be to reverse this erosion of confidence. To achieve that, the Europeans should confirm that the recent signs of an increase in defence spending will be sustained and used to make real improvements in capabilities of benefit to both European and Nato operations. The US could usefully reaffirm that even in a world of great-power competition, the alliance is a precious asset, and one that differentiates the free world from authoritarian states. That’s the birthday present Nato needs and deserves.

• Peter Ricketts is a former permanent under-secretary at the Foreign Office and ambassador to France