Profile: Iain Duncan Smith
"Do not underestimate the determination of the quiet man," Iain Duncan Smith declared as his then leadership of the Conservative Party faced grumbles in 2002.
The following year at the party's conference, he said: "The quiet man is here to stay and he's turning up the volume."
As it turned out, he didn't last that much longer as leader, but in the years that followed he bounced back to become a respected and influential figure in the party and the government.
Whatever you think of his welfare reforms, there is no arguing with their ambition, or with his passion and dedication to the brief as seen from his resignation letter to the prime minister.
Yes, there must already have been tensions, given that he was one of the handful of cabinet ministers to declare they were campaigning for the UK to leave the EU in the forthcoming referendum, against the side favoured by David Cameron.
But his resignation, at the culmination of a 48-hour post-Budget row about plans to cut back the amount being planned to be spent on a disability benefit, still came as a bolt from the blue at Westminster.
But who is Iain Duncan Smith? Here is a look at his background, written by Mary Ann Sieghart for BBC Radio 4's Profile programme in 2010:
At the 2010 Conservative Party conference, Iain Duncan Smith, the Work and Pensions Secretary, was introduced as a round peg in a round hole. The man who was a failure as party leader had at last found his niche.
His department accounted for about a third of public spending, and welfare reform was at the heart of the government's plans to tackle the deficit and balance the books.
He had not only reinvented himself, but had won the admiration of his party.
It was Iain Duncan Smith, or IDS as he is widely called, who was credited with moving the Conservative Party back to the centre ground on poverty and public services.
This was not what you might have expected from a pin-striped former Guards officer on the right of his party.
So what sort of man is he?
The Tory MP Bernard Jenkin, who was his campaign manager when he became leader in 2001, points out that IDS is "not emotionally available. For someone who is so closely associated with compassion he is personally very old-fashioned you might say".
But IDS can be funny. He is a talented mimic and - as befits the youngest of five children - is happy to be teased.
But he does sometimes rub people up the wrong way.
Mr Duncan Smith, now 61, had a very stable childhood with an old-fashioned and Christian upbringing.
His father was a highly-decorated fighter pilot who downed 19 enemy planes during World War Two. His mother was a ballerina and his great-grandmother was Japanese, from Samurai stock.
His most formative memory is of watching his much-admired father with tears streaming down his face at Winston Churchill's funeral. He had never seen his father cry before.
At 14, Mr Duncan Smith was sent to HMS Conway, a boarding school on Anglesey for boys aiming to join the Navy.
Clive Plummer was a schoolmate and remembers that: "We didn't have any cleaners, so brushing parade decks, cleaning classrooms and dorms were all managed by the cadets. So in managing teams, you would make sure that not only were the most able utilised but also the less gifted.
The young Iain was not particularly academic, but he was very sporty. He won the national schools' triple jump and came second in javelin.
He later joined the Army and went to Sandhurst, but not university. Michael Mitchell met him while he was serving in Germany and says that he was not like your average Guards officer. He read widely, wrote poetry and painted.
He was also mischievous.
"He was best man at my wedding in Belgium," Mr Mitchell said.
"At Belgian weddings the bride, groom and parents have to stand in a line and receive guests for two, three hours, and I had a call of nature and asked Iain to stand in my place. And when people asked: where's Michael? He'd say - 'ooh, the last time I saw him he was walking down the road with his suitcase', or he'd say: 'Madam, are you sure you're at the right wedding?' Typical Iain - wanting to inject fun into something."
But no-one then thought he had political ambitions.
IDS left the Army against his father's wishes after just six years, and joined the defence company GEC-Marconi. His Conservative Party biography claimed he was a director, which was not correct. And his own biographical notes said he studied for a degree at Perugia University, when he only did a language course in that city.
In 1982, he married Betsy, daughter of Lord Cottesloe. They have four children.
IDS entered the Commons in 1992, in Norman Tebbit's old seat of Chingford and Woodford Green.
As a Eurosceptic, he rebelled over the infamous Maastricht treaty, knowingly risking his chance to get a promotion to government. He told his former press secretary Nick Wood: "I'll fight for what I believe and if I don't get a job - so be it."
The then Prime Minister John Major never did give IDS a job, but when William Hague became party leader he appointed the Maastricht rebel to shadow defence and then social security.
It was a surprise to everyone when he ran for the Tory leadership in 2001. It was perhaps even more of a surprise that he won, having reached the run-off by only one vote.
He defeated the runner-up, former Chancellor of the Exchequer, Kenneth Clarke, only because he was more sceptical on Europe.
He really was the accidental leader. Tim Montgomerie, who went on to run the Tory activist website, ConservativeHome.com, wrote speeches for him and eventually became his chief of staff. He says that, as leader, IDS was "completely unprepared. He had no staff, no infrastructure, no worked-out agenda".
Mr Duncan Smith knew that the Tories had to shake off the image of being the so-called "nasty" party. His response was to shift the focus towards public services - like health and education - and tackling poverty.
A formative experience was a trip to the sink estate of Easterhouse in Glasgow.
Mr Montgomerie remembers that "on that cold grey day when he visited Easterhouse and was taken round by a Baptist minister, and stopped in a stairwell where he saw paraphernalia of drug abuse next to a child's teddy - something came together.
"Something suddenly clicked," Mr Montgomerie adds. "He realised here was his personal mission and a mission for the Tory party."
His problem though, was that he was struggling to connect with the political establishment.
He did try, such as when he told the Tory conference in 2002: "There are those who do not know me yet, who will come to understand this. When I say a thing I mean it. When I set myself a task I do it. When I settle on a course I stick with it. Do not underestimate the determination of the quiet man."
Labelling himself "quiet" was not considered a masterstroke in political rhetoric.
Stuart Wheeler, then one of the Tory party's biggest donors, threatened publicly to withdraw his funding unless IDS went.
"As leader I think he was a disaster," he said.
"He lacked gravitas and came over as weak. He was a bad communicator. One read that he was high-handed with his staff and that his office was shambolic. He'd lose important papers down the back of sofas."
In the end, Mr Duncan Smith's own MPs forced him out through a vote of no confidence.
Enoch Powell once said that all political careers end in failure, but for Duncan Smith, failure came in the middle of his career instead.
For after losing the leadership, he decided to devote himself to improving the lot of the poor.
He set up the Centre for Social Justice, whose work became the cornerstone of Conservative policy on welfare reform, and which he himself is now putting into practice in government. It has already caused controversy, with opponents claiming the poor will be hardest hit by the government's spending cuts.
But it remains his belief that the best way to alleviate poverty is to get people off benefits and into work. By all accounts it was a huge surprise to him to get the call offering him the Work and Pensions Secretary job after the 2010 election.
There has always been a sense of tension between his department and the Treasury - he had to win a bruising battle with the Treasury right at the start to get the go ahead for universal credit.
The on-going effort to introduce Universal Credit has been a constant while he's been in his job. His resignation cites the "enormous strides" made but suggests that those tensions with the Treasury have been more heated than thought.