In Havana last March, when Pope Benedict sat down with Fidel Castro, the revolutionary leader jocularly asked his fellow octogenarian: "What does a pope do?"
Benedict proceeded to tell Castro, who had stepped down as president in 2008 for health reasons and had to be helped to walk into the room, about his duties as leader of the 1.2 billion-member Roman Catholic Church.
Little did Castro know that Benedict was himself contemplating retirement.
A pope has not abdicated in some six centuries, and the Catholic faithful have come to expect the man whose titles include successor of St. Peter and "servant of the servants of God," to stay in office until his dying breath. His decision to take that step, just under a year later, would shake the foundations of a Church already reeling from a series of scandals - from problems at the Vatican Bank to allegations of sexual abuse - and facing challenges to its authority around the world.
Back home in the Vatican in the weeks after his Cuba visit, Benedict spent time in the prayerful silence of his small private chapel in the Vatican's Apostolic Palace where a large bronze Christ on a crucifix looks down from a wall. At some point last spring he decided he should go.
"The pope's decision was made many months ago, after the trip to Mexico and Cuba, and kept in an inviolable privacy that nobody could penetrate," wrote Gian Maria Vian, editor of the Vatican newspaper l'Osservatore Romano.
The current pope has never been as well-loved as his charismatic predecessor John Paul, who died in pain because he felt he should "not come down from the cross". Benedict's decision has some faithful asking if Benedict was the right person for the job in the first place.
It was at least partly borne of his own physical shortcomings. While he was in Mexico on the first leg of his March trip, he lost his balance in his residence, hitting his head on a bathroom sink. The accident was kept secret until the Vatican confirmed it last week, but insiders say it reminded Benedict of his encroaching age and physical frailty. The pope was fitted with a pacemaker years ago, the Vatican also disclosed.
Reuters has spoken to cardinals and other Vatican insiders and Church experts to delve into Benedict's thinking and get an idea of how he made his decision to step down. Most sources spoke on condition of anonymity. The picture they paint is of a serious intellectual who let himself become isolated in the Vatican, ill at ease with the day-to-day running of the Church.
Pope John Paul wore his accidents, his hospitalizations and his diseases like badges, believing they could inspire others who were suffering. But Benedict is a different type of man.
"This is a man of incredible privacy," said a Vatican official who has known him for many years. "He had very few friends."
"He certainly did not consult widely," said another Vatican official. "You cannot consult widely in the Vatican without it leaking. It might have been to a very restricted group, perhaps posing the question hypothetically."
On May 23, 2012, less than two months after his meeting with Castro, Benedict faced an event that would shake his confidence and reinforce his still-secret decision.
The Pope's personal butler, Paolo Gabriele, was arrested and charged with leaking sensitive documents from the pontiff's desk to the media. The documents alleged corruption in the Vatican and sparked a scandal that cast a rare and unwelcome public light on the inner workings of the Holy See.
Gabriele, one of fewer than 10 people who had a key to an elevator that led to the pope's private apartments, was convicted last October and released from jail after Benedict pardoned him three days before Christmas.
The betrayal had a devastating effect on Benedict, according to an official who knows him well. The Vatican tried to put a good face on the affair, stressing the pope's benevolence towards his betrayer. But the mood in the Apostolic Palace was different.
"He was never the same after that," one official source said of the treachery by someone Benedict considered a son. "It was like shooting Achilles in the heel."
There were other worries on Benedict's mind last year, insiders said.
The Vatican Bank, for decades tainted by scandals, found itself mired in fresh controversy, this time over an Italian investigation into alleged money-laundering.
A group of American nuns, disciplined by the Vatican for being too liberal on issues such as homosexuality, was enjoying a groundswell of popular support, their backers accusing the Vatican of excessive rigidity.
Fresh allegations of sexual abuse committed by priests continued to emerge, in Philadelphia and elsewhere in the United States. In the once staunchly Catholic country of Ireland, the deputy prime minister demanded the resignation of the head of the Church, Cardinal Sean Brady, over his handling of abuse cases.
And despite the pope's strong condemnation of it, gay marriage was making advances in the United States and some Catholic European countries.
At the same time Benedict's health was deteriorating.
Peter Seewald, a German journalist who wrote a book with the pope in 2010 in which Benedict first publicly floated the possibility of resigning, visited him at the end of 2012 while working on a new biography.
"His hearing had worsened. He couldn't see with his left eye. His body had become so thin that the tailors had difficulty keeping up with newly fitted clothes ... I'd never seen him so exhausted-looking, so worn down," Seewald wrote in the German magazine Focus after Benedict announced his abdication.
"I think he simply decided that the forces that were mounting against him were too great and the forces on which he could rely were too meagre to counter this," said the Vatican official who knows him well.
PRIVATE AND ISOLATED
Towards the end of last year came a hint that 2013 would be different. The Vatican usually gives journalists an unofficial indication of how many international trips the pope plans in the following year.
Last year the only trip confirmed was to Brazil in July, for the Roman Catholic Church's World Day of Youth, a kind of "Catholic Woodstock" that can take place with or without the pope.
Benedict had already decided that he would not be there. The only thing left to do was announce his momentous news to his aides and to the world.
In September the pope travelled to Lebanon and in November, with much fanfare, Benedict joined Twitter, attracting more than 1.5 million followers in just a few days. Christmas, New Year and Epiphany came and went with all the pomp and pageantry that only an institution like the Vatican can offer.
Benedict, a stickler for liturgical precision, did not want the Church to be devoid of a visible leader for any of its most important feasts, insiders say. He timed his announcement for a liturgical lull, so a new man could be in place before the start of Holy Week on Palm Sunday, which falls on March 24 this year.
He broke the news to cardinals just after 11:30 a.m. on February 11. It was a regularly scheduled meeting to announce new saints, and most cardinals' minds were probably wandering, according to several who were there.
"People were thinking of their next appointments, at least I was," said one participant.
Then Benedict dropped the bombshell.
"Both strength of mind and body are necessary (to run the Church), strength which in the last few months, has deteriorated in me to the extent that I have had to recognize my incapacity to adequately fulfil the ministry entrusted to me," he told them in Latin.
Benedict had written the 350-word statement himself and before reading it sent it to a Latin expert in the Secretariat of State to make sure the grammar was correct, according to a source familiar with the event.
He read his note in a steady voice with no outward sign of emotion.
"For a few minutes after we understood what had happened, no one moved," said one cardinal. Another said: "I just left in a daze without uttering a word to anyone."
Although the official line is that the pope was "courageous" in making his decision, in private conversations officials repeat two words more than any others to describe Benedict, and how he came to the decision: private, and isolated.
One Vatican official who "respects but disagrees with" Benedict's decision said the pope had become isolated but had also isolated himself.
"Because of his privacy, he was not an easy person to help," the official said. "This was his decision."
Several insiders said they believed part of the decision lay in the fact that the pope never made the full transfiguration from Joseph Ratzinger to Benedict XVI.
"I don't think that he ever really internalized being the pope. He never made that transition where the previous person, that individual, is gone and now you are the pope, that's all you are," one official said.
One sign of this, two Vatican sources noted, was that Benedict continued writing his books using two names: first, Joseph Ratzinger and beneath, Pope Benedict XVI.
"The pope cannot publish private books ... the pope does not have a private person," one official said. "Maybe because he was already too advanced in age, maybe because Joseph Ratzinger was already too substantial a person."
Long before he became a bishop, Joseph Ratzinger was a towering theologian, a university professor known around the Catholic world for his dozens of books and ground-breaking, thought-provoking lectures.
As a young priest he was an "expert" called to assist cardinals at the 1962-65 Second Vatican Council, which attempted to bring the Church into the modern world with liturgical changes and outreach to other religions.
"For experts, Ratzinger was a pioneer in a movement known as 'ressourcement', trying to return Catholicism to its original sources such as the Bible, the Fathers of the Church, and its liturgy," said John Allen, author of several books on Benedict.
Benedict continued in the professor mould as bishop, cardinal and even when he became pope. This was painfully clear in 2006 when he delivered a weighty lecture on "faith and reason" at the University of Regensburg in Germany, where he once taught theology.
Benedict quoted a remark by a Byzantine emperor who linked Islam and violence, offending Muslims around the world. He apologized but still seemed surprised at the power his words carried.
One Vatican official, speaking privately, speculated that the conclave to elect Benedict's successor may discuss whether the new pope should promise not to write "private books" but only papal documents.
In his abdication statement, Benedict concluded that it had become impossible for him to continue being pope "in today's world, subject to so many rapid changes and shaken by questions of deep relevance for the life of faith".
Many inside and outside the Vatican wonder why he did not put better governance in place in the Curia, the Vatican's central administration, to help ease the load on his mind and body.
Critics, such as leading Italian Vatican expert and author Sandro Magister, say the pope put people in positions of administrative power because he knew them and felt comfortable with them rather than for their abilities.
One Vatican official said he believed the Curia "let the pope down" by not preventing problems. In particular, some Vatican insiders criticize Vatican Secretary of State Cardinal Tarcisio Bertone, Benedict's number two.
"Bertone will probably be remembered as one of the worst secretaries of state in history," said another official.
In confidential cables from the U.S. embassy to the Vatican published by WikiLeaks in 2010, diplomats depicted Bertone as a divisive "yes man" with no diplomatic experience or linguistic skills, who protected the pope from bad news.
At the time, Bertone replied: "I am happy to be a 'yes man' if it serves the Holy Father."
A number of Vatican officials privately say that instead of abdicating and throwing the Church into the unknown, the pope could have cut back on travel and other activities to conserve his strength, limiting himself to major decisions and pronouncements, and delegating more.
"It is easy to understand why an 85-year-old man in difficult conditions may feel terribly tired. But the pope does not need to be a hands-on chief executive if he puts in place a good team, which he could have done at any moment because he is a sovereign," a Vatican source said.
"He could content himself with doing very little except praying ... but because the people he had in place were not adequate, instead of removing them, he removed himself," the source said, adding that he would have tried to talk him out of it.
Many Catholic faithful, from elderly women praying in the pews in New York, to monsignors who work in the frescoed offices on the floor below the papal apartments, share that sense of disquiet and loss.
"The fundamental idea that the papacy does not end until the death of a pope has been eroded. It will take 100 years of popes never retiring for this to become a blip," one Vatican source said.