“Being powerful,” Margaret Thatcher once said, “is like being a lady. If you have to tell people you are, you aren’t.” It’s a lesson that could usefully be learned by Gavin Williamson, the elfin defence secretary who longs to inherit her job.

Like a one-man military band, Williamson spent the Christmas period blowing the trumpet, twirling the baton and banging the drum for Brexit Britain. Leaving the EU, he told the Daily Telegraph, would restore Britain as “a truly global player”, determined once again to “play the role on the world stage that the world expects us to play”. Post Brexit, “Australia, Canada, New Zealand, Caribbean countries” and “nations right across Africa” would “look to us to provide the moral leadership, the military leadership and the global leadership” of which they had been too long deprived.

The fantasy of a world that longs for British “leadership” is a bold claim from a government that can barely tie its own shoelaces. Yet like so many delusions, it has deep historical roots. It has bedevilled British diplomacy since 1945, souring its relations not only with the EU but with the Commonwealth countries to whom Williamson makes his pitch.

At the end of the second world war, Britain’s commitment to global leadership remained undimmed – despite its severe reduction in power. So governments of both parties turned to the Commonwealth as an organisation that “has only to develop its resources to match the power of the United States”.

Yet hopes that the Commonwealth would serve as a proxy empire, retaining the benefits of the imperial connection without its unfashionable colonial regalia, were swiftly disappointed. What had once been a gathering of white, Anglo-Saxon statesmen became less white, less “British” and less deferent to instructions from Whitehall. The result was a backlash against the organisation that anticipated later criticism of the EU.

In the 1970s and 80s, the Commonwealth featured in public debate chiefly as a problem: it was a body that did things to Britain, rather than for it, and which allowed ungrateful foreigners to fleece Britain of cash, interfere in its affairs and take advantage of its immigration laws. Britain, complained a Scottish newspaper, had become “the old convenient milk cow”, wielding “the ever-open wallet on which the sun never sets”. Enoch Powell dismissed the organisation as a “humbug” – while more recently, Boris Johnson joked that its main purpose was to provide the Queen with “cheering crowds of flag-waving piccaninnies”.

As the hopes vested in the Commonwealth began to fade, governments did not adjust their ambitions. Instead, they transferred them to the European Economic Community. It was economic weakness that pushed governments towards membership; but like the shipwrecked children in a Victorian adventure story, who crawl to shore and promptly declare themselves king and queen, policymakers rarely doubted that Britain’s rightful place in Europe was at its head. George Brown, a Labour foreign secretary, urged Willy Brandt in 1967 “to let us in” so we can “take the lead”. His boss, Harold Wilson, wrote privately that “if we couldn’t dominate that lot, there wasn’t much to be said for us”.

For Thatcher, likewise, Europe was simply a new vehicle for Britain’s historic vocation to power: an impulse that had “carried our flags, our trade, our culture and our justice to the corners of the earth”. It was “this traditional outward-looking role”, she declared, that had brought Britain into the EEC, for “where power resides, there must British influence be exerted”.

That view was echoed in the press. The Daily Mail celebrated accession with the headline “Now we can lead Europe!”, while the Sun told readers that membership offered “an unrepeatable opportunity for a nation that lost an empire to gain a continent”.

Such rhetoric continued into the 1990s and beyond. Tony Blair promised in 1997 to “give Britain the leadership in Europe which Britain and Europe need”, and “to use our leadership position” to put Britain “at the centre of international decision-making”. Gordon Brown called his book on the 2016 referendum Britain: Leading, Not Leaving, insisting that “Britain has always led the way when things have been difficult in Europe, and I think it’s time that we were a leader again”. David Cameron, likewise, proclaimed that Europe needed “‘a big, bold, brave Britain at the heart of [its] institutions”.

Ian McKellen as King Lear. ‘Britain is fast becoming the King Lear of the diplomatic world: rampaging insanely around the stage and blaming everyone else for the loss of his kingdom.’ Photograph: Johan Persson

Yet this set an impossible standard for membership of a multinational community. Successive governments persistently refused to re-imagine themselves as an equal partner – and, in the imagination of the Brexit right, if Britain could not be a king, it must be a “vassal”, a “prisoner” or a “colony”.

In consequence, Britain is fast becoming the King Lear of the diplomatic world: rampaging insanely around the stage and blaming everyone else for the loss of his kingdom: “How sharper than a serpent’s tooth it is to have a thankless Commonwealth/continent!”

The liberation that Britain so urgently needs is not from Brussels, but from its own illusions. A lesson in humility may be fast approaching – but a disorderly Brexit would be a cruel teacher. As so often, it will not be the worst delinquents who pay the highest price.

• Robert Saunders is a senior lecturer in history at Queen Mary University of London and author of Yes to Europe! The 1975 Referendum and Seventies Britain