On hearing the news the Japanese had attacked Pearl Harbour on 7 December, 1941, Winston Churchill noted later how appalled he had been before adding that, however, that night he had “slept the sleep of the saved and thankful.” He knew the United States would now enter the War and no matter how long it took in the end, the allies would prevail.

For Churchill that moment was the culmination of a long campaign of persuasion, pleading and cajoling. It had been a long two years for him and for Britain, involving increasingly desperate pleas by Churchill to the Americans for much needed supplies and war materials, which they did provide at ever increasing cost. The strength of the military bond that was forged thereafter is now the stuff of legend. But the hard-headed nature of the politics that surrounds that bond is, now as then, unforgivingly tough.

The 76 years that have passed since 1945 have seen many ups and downs in the relationship between London and Washington. The core of the relationship fundamentally centres on intelligence gathering and military co-operation but the political relationship has been subject to much greater fluctuation.

Clement Attlee was not keen to see Britain embroiled in the Korean War. President Eisenhower, who had lived and worked in Britain as Supreme Allied Commander during the War, felt no inhibition in withholding US support for Prime Minister Anthony Eden’s Suez invasion, leading to its swift ending. Harold Macmillan had a very tough time persuading the Kennedy White House to share nuclear deterrent capability, having to use every ounce of personal persuasion and family connection to prevail. Lyndon Johnson was not pleased that Harold Wilson refused to commit British troops to Vietnam. President Reagan oversaw the invasion of Grenada even though it was a Commonwealth country of which the Queen was head. It took time for Reagan’s White House to unambiguously help Britain after the Falkland Islands were invaded. More recently President George H W Bush adopted a policy of refocussing the US’s primary European relationship on Germany and away from Britain, a position swiftly reversed in the wake of Iraq’s invasion of Kuwait.

Throughout this period, London and Washington maintained close political and diplomatic ties. Not slavish or fawning but realistic and rooted in a tough and practical understanding of realities of the partnership. It did not falter for any length of time when either side did something the other disapproved of. The relationship maintained an essential equilibrium that survived the comings and goings of presidents and prime ministers and provided the cornerstone of NATO and Western security.

George W Bush’s invasion of Iraq and then Afghanistan, with the slavish and unquestioning support of Tony Blair, provided a turning point in the relationship which has had profound consequences on both sides of the Atlantic. American and British public opinion has shifted in terms of what people are willing to believe from their political masters on the subject of military interventions. Politicians trust in their generals in terms of what really is and is not achievable, and the generals trust in politicians, has shifted too. Since Bush and Blair we have seen a succession of US and UK leaders falter over military interventions. In Libya President Obama refused to commit forces in support of Britain and France’s intervention. David Cameron’s inability to win a House of Commons vote to support intervention in Syria was a decisive rebuff and had profound consequences in Washington.

No one who has followed Joe Biden through his very long public career can ever have doubted or misunderstood his resolve to withdraw the US from Afghanistan or when he wanted to do it by. Boris Johnson, who was not involved in the decision to invade nor apparently consulted on the decision to withdraw, has been left picking up the pieces – but pick up the pieces he must. The relationship is too important to allow the two countries to drift apart. There are too many shared vital interests and common causes in places all around the world for Britain and the United States to allow themselves to drift apart. The White House and No 10 need to do much better at improving lines of communication and both President and Prime Minister need to work harder at building their personal relationship.