Undoubtedly Mr Castro would have liked nothing better, but physical frailty, it seems, has had the last word. But, as long as he lives, his shadow will fall over whoever succeeds him. And as long as Mr Castro draws breath, he will be a reminder of how little has changed in this corner of the world since Dwight Eisenhower – the 34th president and first on the Castro contemporaries list – bequeathed to his successor, John Kennedy, a secret plan to invade Cuba that resulted in the 1961 fiasco of the Bay of Pigs.

In his declining years Mr Castro has become, for better or worse, a listed global monument, a relic of the vanished age of Kennedy, Khrushchev and superpower brinkmanship, and of national liberation wars led by revolutionaries in dusty military fatigues. Nearly half a century on he is still wearing the fatigues, even though the revolution had fossilised into a regime sustained primarily by the economic siege imposed by Cuba's giant neighbour to the north.

In power since 1959, he has been the world's longest-serving ruler (although King Bhumibol Adulyade of Thailand, the head of state but not of government, has been around since 1946). The defining reality of the Castro era has been the regime's relations with the US, under leaders from Eisenhower to George Bush Jnr.

In fact, Mr Castro's first contact with an occupant of the White House was cordial enough, a letter the 13-year-old schoolboy sent to Franklin Roosevelt in 1940, asking for a $10 bill. "Never, I have not seen a ten dollars bill green American and I would like to have one of them," he wrote, signing off as "Your friend". In reply, Mr Castro received a pro-forma letter, but sadly no money – and for his ties with the US it was downhill all the way thereafter. Two decades later, his guerrilla army toppled the pro-US dictator Fulgencio Batista, and Cuba's undeclared war with Washington began.

Successive US administrations kept up the pressure, with the exception of Jimmy Carter. But that brief thaw ended with the Mariel boat lift of 1980, as Mr Castro encouraged a mass exodus by sea of 120,000 Cubans to the US (including many hardened criminals and people who were mentally ill) to cope with a domestic political crisis. Relations returned to a chill that not even the demise of the Soviet Union could lift. Under George Bush Jnr, who has further tightened travel and financial restrictions against the island, the climate has become frostier still.

The confrontation, however, leaves most rational outsiders baffled. What is it about Cuba, they wonder, that makes otherwise sane American leaders lose their own sense of reason?

After all, a country of 11 million people, with a GDP of $45bn dollars – equal to 0.3 per cent of that of the US – offers not the slightest conceivable security threat. To be sure, dilapidated Cuba is no benign socialist paradise. Thousands of opponents were executed in the early years of the revolution. Today, Mr Castro's regime holds large numbers of political prisoners, suppresses freedom of expression and otherwise tramples on human rights. But is it that much worse than other countries, from the Middle East to China, which Washington counts as allies? Yet Cuba alone is treated as a special enemy, a source of potential Communist contagion that endangers the hemisphere.

By one (admittedly sympathetic) calculation, Mr Castro has survived 638 assassination attempts by the CIA, by such devices as exploding cigars, poisoned food and an infected diving suit. Every year a farcical vote takes place in the United Nations General Assembly in which it declares its opposition to America's economic blockade of Cuba. The 2007 edition took place last October, when the resolution was upheld by 184 to four. Those voting against were the US, Israel, Palau, and the Marshall Islands. Oh yes, Micronesia abstained.

By any measure, the US embargo has been utterly counterproductive. Not only has it failed to hasten the demise of Mr Castro and the return of democracy. Most dispassionate observers believe the blockade has positively hindered those two goals, by hardening the sympathies of a strongly nationalistic people, and permitting Mr Castro to present himself as a victim of Yanqui imperialism. Quite possibly nothing would do more to undermine the regime than the lifting of all US sanctions.

There are other, wider consequences for the US, and no less adverse. Washington's bullying of Cuba has soured ties with many Latin American countries. It has also fuelled the growth of an anti-US bloc on the continent, spearheaded by Hugo Chavez, President of Venezuela, taunter of Washington and Mr Castro's most devoted foreign friend.

So will US policy now change? There is little immediate sign. Hopes were briefly raised when the Democrats regained control of Congress at the 2006 midterm elections, but those advocating a more liberal approach were disappointed. As for the Bush administration, it repeats the litany of the last quarter century: nothing will change until Cuba itself changes. The embargo, John Negroponte, the deputy Secretary of State, said yesterday, would not be lifted "anytime soon".

But President Bush, as noted, will not be around much longer, and among those vying to succeed him some intriguing policy differences have emerged. The standard wisdom has been that no candidate will stick his or her neck out over Cuba, for fear of upsetting Cuban-American voters, fiercely anti-Castro and concentrated in important states such as New Jersey and above all Florida, decisive in the 2000 presidential election.

But the political equation may be shifting. For one thing, the Cuban-American vote is less monolithic than before. For another, only the blind cannot see the absurdity of existing American policy. In a campaign where the lone superpower's attitude to countries it dislikes – most obviously, of course, Iran – is already being hotly debated, Cuba could yet feature large.

Predictably John McCain, the all-but-certain Republican nominee, is most resistant to a new departure. Mr Castro's resignation, he declared yesterday, was "an opportunity for Cuba" – in other words, only when Cuba has transformed itself should the US transform its policies.

Hillary Clinton adopted a similar, though more guarded, approach. But her rival, Barack Obama, is already on record in support of an easing of restrictions on travel and financial remittances to Cuba, insisting that the time for re-assessment has come. And maybe Mr Castro knows something the rest of us don't. As long ago as last August, he predicted that a Clinton-Obama ticket would be "apparently unbeatable".

I wish to fight on as a soldier of ideas

Dear compatriots,

I promised you on 15 February that in my next reflections I would touch on a subject of interest for many compatriots. This time that reflection takes the form of a message...

I held the honourable position of President for many years... I always exercised the necessary prerogatives to carry forward our revolutionary work with the support of the vast majority of the people.

Knowing about my critical state of health, many people overseas thought that my provisional resignation from the post of President of the Council of State on 31 July 2006, leaving it in the hands of the First Vice-President, Raul Castro, was definitive. Raul... and my other comrades in the party leadership and the state, were reluctant to think of me removed from my posts despite my precarious state of health...

Preparing the people for my psychological and political absence was my primary obligation... I never ceased to say we were dealing with a recuperation that was "not free from risk". My desire was always to carry out my duties until my final breath...

To my close compatriots... I tell you that I will not aspire to or accept... the post of President of the Council of State and commander-in-chief.

The path will always be difficult and will require the intelligent strength of all of us... "Be as prudent in success as you stand firm in adversity" is a principle that must not be forgotten. The adversary we must defeat is extremely strong, but we have kept him at bay for half a century.

I do not bid you farewell. My only wish is to fight as a soldier of ideas. I will continue to write under the title "Reflections of Comrade Fidel". It will be another weapon in the arsenal on which you will be able to count. Perhaps my voice will be heard. I will be careful.

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