‘She promised to follow him to the end of the earth,” ran the famous spoof Gone with the Wind poster of Ronald Reagan cradling Margaret Thatcher as a mushroom cloud blossoms in the background. “He promised to organise it.” But relations between the two right-wing leaders were not always as cosy as we were led to believe: natural sympathy masked intense rivalry, and if there were smiles in public it was because she had, as usual, come out on top. “She wore away at Reagan,” the political scientist David Runciman wrote recently in the London Review of Books. “She was always much better informed than he was and able to dominate any discussion. Sometimes he barely got a word in.”

So if Reagan wanted to get his way, the best approach was to stay mum. As cabinet papers published this week reveal, the British Prime Minister knew nothing at all about the US invasion of Grenada, the small Caribbean island and British crown colony, until it was already under way.

On 24 October 1983, Foreign Secretary Geoffrey Howe made a statement to the House of Commons about the island, where five days before the Marxist prime minister Maurice Bishop had been overthrown and assassinated by what a history of the West Indies calls “a small ultra-Leninist clique” inside his government. But Howe made no mention of a possible invasion – because he knew nothing about it. That evening, Mrs Thatcher had dinner with the US ambassador. All was sweetness and light, it appeared, despite the 700-strong Cuban force that was building a large international airport in the island 1,500 miles south-east of Miami.

Later that evening, Washington sent a cable to London, informing its Special Relationship buddy that the US was considering a request from some Caribbean nations to intervene in the island. It followed up with another message, reporting laconically that the invasion was actually under way.

Mrs Thatcher exploded. All the “dear Ronnie” chumminess was forgotten. “I must tell you that the decision which you describe causes us the gravest concern,” she fired back. “I cannot conceal from you that I am deeply disturbed… [I] hope that even at this late stage you will take [my advice] into account before events were irrecoverable.”

As she learned when they spoke on a secret phone line, the Rubicon had already been crossed, or, as Reagan put it: “We are already at zero.” Within hours, 6,500 marines had descended on the island. It was over in days. Thatcher, as Howe confided, was “deeply disturbed” by this “humiliation” which caused her “intense embarrassment”.

The embarrassment and humiliation have, in fact, been known for years – Thatcher was frank about them in her memoirs. But only now do we see how carefully Washington kept its supposedly close ally in the dark over plans which, as British officials noted bitterly, had been in development for a long time.

The relationship had, of course, been profoundly unequal for decades; it was only through guile and force of personality that Thatcher bent Reagan to her will over the Falklands invasion and thus gave the world the impression that the relationship was far more equal than it really was. But the Grenada debacle exposed more than British weakness; it revealed Britain’s bizarre blindness both to the real dangers of the Caribbean becoming a Soviet lake, and to the improbability, 20 years after the Cuban missile crisis, of Washington standing by and allowing that to happen.

Today, more than two decades after the fall of the Berlin Wall, it is strange to realise how easy-going the Conservative British government was about events such as those in Grenada, where an “ultra-Leninist clique” closely linked to Moscow murdered the elected prime minister. Whitehall probably regarded them all indiscriminately as daft islanders who could readily be brought to heel if need be; in any case, they were thousands of miles away, and doubtless loyal to the Queen. It was a shock for them to realise that the US saw it all rather differently.

A Roman road will be no more

Rome has its share of problems, but its traffic flow is a thing of beauty. Try it on a scooter and you will see what I mean: the fast, swirling, multi-lane Lungotevere roads along the Tiber, the vast free-form roundabout of Piazza Venezia, the incredible ride through the middle of the Roman Forum and up around the Colosseum. The Italian genius demonstrated in some of its superb autostrade produces an exhilarating experience when you ride through Rome.

So it is no surprise that many in the city have reacted in shock and disbelief at plans by the new mayor, Ignazio Marino, to close the Imperial Forum boulevard to private traffic and make it a vast pedestrian zone. At a stroke the exhilaration of zipping through the world’s most famous ancient monuments will be killed; traffic in the rest of the system risks permanent gridlock.

Yet just writing those words I recognise how selfish they must sound: what is exhilarating for the scooterist turns this unique zone into a nightmare of noise, smoke and danger for those on foot. No one could deny that this area – cleared of medieval tenements by Mussolini – is unique. Mr Marino’s plan is, by banning private traffic from Piazza Venezia to the Colosseum, to transform the area into “a walk into history”. It is an idea whose time has come.

Twitter: @peterpopham