September 2020: Nato began gloriously 71 years ago, with the signing of the Washington Treaty by the august representatives of 12 nations committed to defend each other in perpetuity.

It ended ignominiously last Thursday with the padlocking of the gate at Nato's Brussels headquarters by a Flemish security guard named Karel van Aachen.

Technically, the organisation still exists. The treaty is still in force; the 28 members of the alliance are still pledged, in theory, to defend each other against aggression; think-tank conferences continue to endlessly debate "whither Nato" in ornate assembly halls; Georgia still publicly holds to its ambition of joining the alliance in some distant future.

But long before Nato Secretary-General Gerhard Schroder abandoned his nearly empty HQ last month, it was clear to all observers that, over the course of just a few years, Nato had gone from the strongest and most successful alliance in history to an empty shell and an irrelevance.

It was destroyed not by Russian armies but by a lack of interest from its members. The story of Nato's demise demonstrates that sometimes alliances end not with a bang but with a whimper.

THE LONG WHIMPER

In Nato's case, the long whimper of its demise began with the inauguration of US President Donald Trump in January 2017.

Throughout the endless 2016 presidential campaign, Mr Trump had railed against American allies that he felt did not carry the burden of their own defence. He hinted darkly that, as president, he would not defend allies that did not pay their share. His praise of Russian President Vladimir Putin further stoked fears in Eastern Europe that he would abandon them to Russia's tender mercies.

Once he became president, Mr Trump's attitude towards Europe and Nato became just as erratic as his ramshackle presidential campaign. He appointed Cabinet secretaries who praised Nato in their confirmation hearings. He allowed visiting British Prime Minister Theresa May to assert that he "supported Nato 100 per cent".

Then, just as suddenly, he would veer back towards bashing allies, calling Nato obsolete or attacking the European Union as a German plot.

Each new tirade would be followed by a new round of tumult in the press and hand-wringing on the part of Europeans. And yet little changed on the ground. US forces remained in Europe, US planes took part in patrolling the skies over the Baltics and US soldiers still participated in Nato military exercises. Beneath the headlines, Nato quietly remained, on paper, the most powerful military alliance in the world.

At first, far from breaking the alliance, Mr Trump's threats even appeared to motivate Europeans in a way that the blandishments of previous presidents had not managed. European defence spending crept up towards their commitment of 2 per cent of gross domestic product, and Europeans established new mechanisms for defence cooperation within the post-Brexit EU. At Mr Trump's insistence, Nato proclaimed that counter-terrorism was its primary mission and embarked on multiple studies to explore how Nato might fulfil its new purpose.

In the end, Nato's new mission did not shift much in the way of resources; the alliance simply stopped talking about its previous core mission of defending Europe from Russian aggression.

But these cosmetic changes allowed President Trump to claim that he had succeeded in adapting the alliance to his "America first" philosophy.

In a famous speech delivered in front of the Las Vegas facsimile of the Eiffel Tower, he proclaimed that "now, instead of America working for Nato, Nato works for America". Mr Trump no longer thought that Nato was obsolete.

To the contrary, it became for him a symbol of how he could restructure American alliances to serve American purposes.

ROTTEN TO THE CORPS

But beyond the symbolism, it was not really clear that Nato worked for anyone any more.

When Russia stepped up its proxy war in Ukraine in mid-2017, Nato debated a response, but with US energies focused on building a wall on the country's southern border, it failed to find any consensus for new sanctions or for reinforcing existing deployments in the east.

Poland, France and Germany decided that the EU's new Permanent Structured Cooperation mechanism for defence was more fit for this purpose. Along with most of their European partners, they began using it to supply weapons and training to the Ukrainian government. Officially, Nato declared its neutrality. Russian propaganda pivoted away from denouncing the United States and Nato and towards excoriating Germany and the EU.

In early 2018, the Egyptian economy went into free fall, and the government collapsed. As disorder reigned in Cairo, hundreds of thousands of refugees began appearing on the shores of Greece and Italy. Once again, Nato considered action - in this case, a naval mission to intercept and return the refugee boats, seen as both a humanitarian and protective measure, similar to the missions it conducted in the Mediterranean in 2016. But this time, Eastern European members, stung by Nato's neutrality in Ukraine, opposed the alliance's participation in the effort. The US, embroiled in a scandal over former Fox News chief executive Roger Ailes' purchase of The New York Times at an Internal Revenue Service tax auction, did not take sides, and in the end Nato did nothing.

Then, in January 2019, in response to the US decision to search Iranian ships in the Persian Gulf for weapon shipments, Teheran staged a coup in Baghdad.

The Iranian puppet regime ordered US forces out of Iraq while combined Iraqi-Iranian forces attacked Turkish forces in Iraqi Kurdistan and began arming the Kurdish insurgency in south-eastern Turkey.

Turkey, supported by the Trump administration, asked Nato to invoke its sacred Article V - that is, to declare Iran's actions as aggression against a Nato member - and go to Turkey's aid.

Most of the European members of Nato, including France, Germany, Poland and Italy, flatly refused. Interestingly, these countries had met their 2 per cent defence commitment and even endorsed the Nato turn to counter-terrorism. But they refused to use their new-found defence muscle to oppose what the US and Turkey saw as Iranian "terrorism" in northern Iraq and south-eastern Turkey.

Nato's refusal to respond to an Article V request triggered the resignation of Nato Secretary-General Jens Stoltenberg. In his resignation letter, he noted that there was little reason to continue running an organisation that could not or would not respond to its members' needs.

Many in Europe agreed that Nato's time had passed but the US and Britain were not ready to give up. With support from Germany's Social Democratic-led grand coalition government, they found in former German Chancellor Gerhard Schroder a compromise candidate for secretary-general to take up the challenge of redefining Nato for a new age, though just what that meant was left unclear.

The Russians welcomed his appointment and declared that they no longer opposed Nato membership for Montenegro and even Serbia. They further said they would consider joining the alliance eventually, an announcement that the Trump administration publicly praised.

Russian support, however, did not help Mr Schroder reverse the decline. Most European members, outraged by Nato's good relationship with their enemy in Ukraine, sent their scarce staff officers to EU commands.

US attention was absorbed by Mr Eric Trump's trial on charges of insider trading and the Turkish-Iranian war. US officials often did not show up at Nato meetings, and large-scale military exercises just stopped happening.

Most countries quietly ceased to even contribute to Nato's common budget, diverting the funds to their unilateral immigration patrols in the Mediterranean or military training in Ukraine.

Struggling even to keep the lights on, Mr Schroder got US and Russian support in 2020 to relocate Nato HQ to a former military base in Bulgaria in what he hoped would eventually be the geographic centre of the alliance.

Europeans did not object but most of Nato's staff did not even bother to follow him there.

IT'S THE SOLIDARITY, STUPID

In retrospect, it is clear why Nato faded away. For decades, Nato members had focused on what divided them. They had argued mightily over burden-sharing and how to respond to Russian aggression or to disorder in North Africa. These were immensely important issues but the disputes distracted attention from what made Nato special: the deep commitment of its members to each other's security.

Of course, Nato members did not always agree on what its priorities should be but Nato, as a whole, took seriously the threats that each individual member saw to national security.

As a result, in most of its 70 years, Nato, far from being obsolete, had been the tool that US and European policymakers turned to in crisis after crisis. In the Cold War, in the Balkans, in Afghanistan, in Libya and elsewhere, US presidents and European leaders had found that Nato provided not just military capacity but also a mechanism for rallying allies and securing broader legitimacy for their own defence priorities.

Nato worked because its members believed that their partners had their back. Solidarity was at its heart. It is easy to see now that President Trump solved Nato's burden-sharing dilemma - by destroying its solidarity.

In putting America first, and failing to pay attention to their problems, he got his allies to pay more, but he also guaranteed that they would care less. A Nato that was built to work primarily for America no longer worked at all.

And so America's European allies are not with it in its current struggle with Iran, just as America is not with them in Ukraine.

Looking back at Nato's years of achievements, this seems a shame. But absorbed as we are with the new world disorder, nobody seems to care. Mr van Aachen, the security guard who closed Nato headquarters, was asked recently what he did with the key to the formerly glorious building.

"I think it's at home in my top drawer," he admitted. "Nobody asked me for it."

WASHINGTON POST

•The writer is the director of research at the European Council on Foreign Relations and a non-resident Senior Fellow at the Brookings Institution.