As President Donald Trump ramped up his campaign to ‘acquire’ Greenland, he encouraged speculation that he might even use military force. The crisis was defused, at least for now, by some deft Davos diplomacy. But the brutal pressure Trump applied to Denmark increases the risk that, by the end of his presidency, the trust which has underpinned NATO since it was founded in 1949 will have been destroyed.

What would then happen to the organisation, with its 10,000 staff, its military commands and multiple agencies? History suggests that security structures can prove surprisingly unwilling to fade away. They can linger on for decades after they stop being relevant. That was certainly the case with the Western European Union (WEU).

This was the brainchild of Ernest Bevin, Foreign Secretary in the 1945 Labour government. With Germany defeated and occupied, he championed mutual defence pacts against the threat Stalin posed to European security. He first corralled his French and Benelux colleagues into signing the 1948 Brussels Treaty on Western Union (WU). This was partly a hedge against German rearmament, still a major French preoccupation at that time. But the Treaty also committed members to ‘consult on any situation which may constitute a threat to peace in whatever area that may arise’.

The Western Union was not long the flag-bearer for European security. Bevin had a much more ambitious goal: to keep the Americans committed to the defence of Europe as the best way of deterring Stalin. He used the Brussels Treaty as a stepping stone to a transatlantic partnership with the US, Canada and a wider spread of Europeans. Twelve nations signed the Washington Treaty in 1949, establishing NATO. The organisation was soon given real clout with an integrated military structure headed by a US Supreme Allied Commander based in Europe.

The Western Union could have been wound up at that point, but it continued to exist and proved useful in 1954 to the then Foreign Secretary, Anthony Eden. He used it as part of a complex set of diplomatic manoeuvres to end the occupation of Germany and embed the new Federal Republic of Germany (FRG) in European security structures. To achieve that, the FRG signed a revised version of the Brussels Treaty. The organisation was renamed the Western European Union (WEU) and a parliamentary assembly was created. This opened the way to the FRG joining NATO in 1955.

From then on, the WEU was an organisation without a purpose – except that it was the only forum where European countries could discuss defence issues among themselves. The European Economic Community established in 1957 did not deal with security and defence issues. The WEU therefore drifted on, a ghostly presence as NATO took the strain of deterring the Warsaw Pact. The WEU Council met monthly in a gloomy building in Grosvenor Gardens in London for desultory discussions on the security situation and a WEU Institute was established in Paris.

The fall of the Berlin Wall in 1989 was an earthquake that transformed the European security landscape, and it briefly revived the fortunes of the WEU. Responding to the new situation, the Treaty on European Union, agreed at Maastricht at the end of 1991, created the Common Foreign and Security Policy, and contained a Delphic reference to the objective of common European defence.

Any suggestion of a role for the EU in defence would have been anathema to Margaret Thatcher, conscious that this would be seen in Washington as duplicating and potentially weakening NATO. But by then John Major had replaced her, and he was a shade more pragmatic. He realised that the momentum towards an eventual role for the EU in defence was unstoppable, but wanted to delay it as long as possible.

I was working on security policy in the Foreign Office at that time. Playing for time, we came up with the idea that the EU could outsource European defence to the WEU. We persuaded the other WEU members to sign a declaration, published the day after the Maastricht Treaty was agreed, which defined the future role of the WEU as ‘the defence component of the European Union and a means to strengthen the European pillar of the Atlantic Alliance’.

For a fleeting moment, the WEU came to life. Military exercises were organised and in early 1997 the WEU made its only operational deployment, sending a Multinational Advisory Police Element to Albania for six months following riots there. But it was not to last. In 1998, Tony Blair and Jacques Chirac signed the St Malo Declaration, which reconciled long-standing differences between London and Paris on the EU’s role in defence. The two leaders agreed that the EU should develop its own military capability, designed to complement, not compete with, NATO. St Malo was the basis for all the EU defence structures which grew up in the years after that, and which led on to a number of EU military operations.

This was also the death knell for the WEU, though even this final phase of its existence was drawn out. Over the following decade, WEU functions were progressively absorbed into the EU. It was only in 2010 that the UK and other members announced their intention to withdraw from the Brussels Treaty. The WEU parliamentary assembly was determined to keep going as long as possible, but it, too, finally sank below the waves in 2011.

NATO is an incomparably larger and more significant organisation than the WEU ever was. It is entirely possible that, by the end of the Trump presidency, NATO will no longer be the basis of Europe’s defence. The WEU experience suggests that the organisation would continue to function, perhaps for decades, occasionally proving a useful forum, but no longer the place where difficult decisions were made. The Europeans and Canadians would develop other structures to protect their vital interests. Over time, staff numbers would fall, and functions cease. But 50 years later, the NATO Parliamentary Assembly might still be soldiering on.