When the UK’s new aircraft carrier sets sail on its first operational deployment this year, the Royal Air Force/Royal Navy aircraft on the flight deck of HMS Queen Elizabeth will sit alongside a squadron of US Marine Corps jets. An American destroyer USS The Sullivans will join the escort. So configured, this naval strike group offers a useful metaphor for the “special relationship”: Britain as both partner and willing prisoner.

The Brits say the operation underscores an unmatched level of military collaboration. The claim is hard to gainsay. Senior Washington figures confirm that the co-operation established during the second world war is deeply embedded in mutual trust.

For all that, the carrier group illuminates the unevenness of the relationship. The marines are there for a reason. Shortage of cash has forced the UK to cut its order of F-35 jets from the US. The Marine Corps planes fill an otherwise embarrassing gap. Likewise, USS The Sullivans. The Royal Navy lacks sufficient warships to defend the carrier properly and meet commitments elsewhere. Boris Johnson had more pressing concerns after Joe Biden’s presidential victory. The UK prime minister, an at times fawning supporter of Donald Trump, feared Mr Biden would shun him, and was relieved to receive his telephone call. Silly as it seems, it mattered that Mr Johnson was the first European leader to hear from the new president.

Mr Biden’s decision to let bygones be bygones reflected in part his generosity of spirit. Yet it was also testimony to Washington’s hardheadedness. From Winston Churchill on, Brits have invested the relationship with an almost spiritual significance. For the Americans, it has been strictly business. Trawling through US government archives to research a new book, what struck me was the consistency with which presidents both valued the alliance and, when it suited, broke with it.

Thus Dwight Eisenhower’s administration voted with the Soviet Union at the UN in condemnation of Anthony Eden’s 1956 Suez expedition. Richard Nixon shut down intelligence and nuclear co-operation when Edward Heath was judged to be ganging up with Europe. Ronald Reagan hesitated before backing Margaret Thatcher’s war to retake the Falklands. There was no malice in any of this. The US simply asserted its national interest.

The asymmetry is also apparent in the terms under which the US supplies Britain’s nuclear deterrent. The British insist their Trident missile system is wholly “independent”. The small print says otherwise. Washington requires that the system operates under Nato command, unless “supreme national interests are at stake”.

Harold Macmillan’s cabinet worried about this when the bargain was first struck in 1962. And, as decades have passed, the US has taken control of missile maintenance and servicing. As a former UK ambassador to Washington told me, Britain is dependent on the US for its “independent” deterrent to work.

None of this gives cause to suggest that, having blown up the EU pillar of its foreign policy, Britain should break with the US. The danger, though, is of a slide into ever deeper dependency — that, post-Brexit, the relationship becomes still less a partnership and more a prison. Britain’s armed forces already look like a bolt-on to the US military.

Germany and, dare one say it, France remain strong US allies while asserting what Brexiters call “sovereign” independence. Their relationships with Washington are healthier for the absence of the neediness apparent in London. Mr Biden, incidentally, took the trouble to refer to France as America's “oldest ally”.

Brexit demands a fundamental reappraisal of UK security and defence interests — and of the spread of alliances and capabilities it needs to safeguard them. Close collaboration with European allies, if not the EU, will inevitably be part of this, as will the relationship with Washington. A clear-sighted assessment, however, would also ask if the UK could not deploy its diplomatic, intelligence and military strengths to make a more distinctive contribution to international security. A lead role in post-conflict stabilisation programmes?

The other day, I heard a leading intelligence expert explain how national security is increasingly defined by technological edge in digital communications and artificial intelligence. Comforting as they can seem as emblems of national prestige, it was hard to see how hugely expensive nuclear missile systems and large, vulnerable, aircraft carriers match that threat.

The writer’s new book ‘Britain Alone: the Road from Suez to Brexit’ is being published Thursday