There are two ways to drive inland from Freetown. The first is to go through the eastern, poorer quarters of the Sierra Leonean capital. There decrepit vehicles jam narrow streets lined with mouldering clapboard houses. With such heavy congestion, it can take many hours to make the journey. The alternative is to take the so-called mountain road. You drive up into the hills, past the camp of the British army-led training team left over from Tony Blair's little war in 2000. Soon the tarmac ends and a dirt road threads past straggling villages into the forest.

The track of reddish laterite – which bypasses the city and its traffic – is treacherous after rain, and traces a route down into a broad valley. A mile or so before it rejoins the main highway leading inland, a side road branches off to the left through a quiet village. At the far end of the settlement stands a faded sheet-metal advertisement for Goodyear tyres. And there, most afternoons, a tall man with close-cropped, greying hair sits on an open porch by the side of the road, often dressed in just a pair of shorts. If you arrive late in the day he may be drinking gin from a plastic sachet. His name is Valentine Strasser; he is 45, and was once the youngest head of state in the world.

It is ten years since the end of the 11-year civil war in Sierra Leone. In 2007 power changed hands at the ballot box – and yet, to the outside world, the iconography of that long war – child soldiers, violent amputations and conflict diamonds – is ineradicable.

The story of Strasser, who seized power in a military coup at the age of 25 in 1992 and ruled for four years until he was deposed by the same method, is unusual even by the experience of West African dictatorships. His improbable rise to executive power and his precipitous fall to roadside penury is a parable of the human consequences of premature kingship.

Strasser says he was born on 15 September 1966 in Freetown. His father was a teacher, his mother a small-time businesswoman. After attending the Sierra Leone Grammar School (founded in 1845), he became an army officer, serving in neighbouring Liberia as part of a regional peacekeeping mission, the Economic Community of West African States Monitoring Group (ECOMOG). Like Sierra Leone, Liberia was established as a colony of freed slaves. Civil war had broken out there in 1989, and in 1991 ECOMOG was attempting to secure order in the capital, Monrovia. "Fighting was going on every corner from three factions," Strasser told me one evening, speaking softly and with a slight lilt.

After seven months in Liberia, he returned home. The war followed him. In March 1991, rebel fighters crossed over from Liberia into the remote eastern part of the country. This incursion of as many as 2,000 men, most of whom were on loan from the Liberian warlord Charles Taylor, marked the beginning of Sierra Leone's decade-long conflict.

Led by Foday Sankoh, the rebels came to be known as the Revolutionary United Front (RUF). Sankoh was a former army corporal and one-time jobbing photographer and, like others among the initial RUF leadership, he had received training at al-Mathabh al-Thauriya al-Alamiya, Muammar al-Gaddafi's World Revolutionary Headquarters in Benghazi, Libya.

By 1991 Sierra Leone was close to ruin. After independence from Britain in 1961, there had been a brief period of relatively functional democracy under the leadership of Sir Milton Margai. He died in 1964 and was succeeded by his less respected stepbrother Albert, who disbursed vital positions in government to people of the Mende tribe regardless of qualifications.

The decline accelerated under Siaka Stevens, a trade unionist who was elected in 1967 but did not become prime minister until the following year because of a series of coups. In 1971, Stevens declared himself president. Charming but spectacularly corrupt, he systematically degraded state institutions and operated a system of personal patronage. He plundered Sierra Leone's diamond wealth and even entered into negotiations with an American company to have toxic waste dumped in the country in exchange for a fee of $25m.

“At the age of 80, Stevens left office with an estimated fortune of US$500m," says Sareta Ashraph, a London-based lawyer formerly at the UN-backed Special Court for Sierra Leone who is now working on a history of the civil war.

"The sheer corruption and violent repression of the Stevens regime extinguished the hopes of an entire generation and laid the foundation for the country's brutal civil war."

Following riots in Freetown, Stevens stepped down in 1985. Two years later, at a ceremony held in the grounds of parliament, a local preacher compared the former head of state's reign to a "17-year plague of locusts" in an address that was broadcast on national radio.

The next president was Joseph Momoh, a military officer. Despite his initial promises of reform, corruption persisted under him. He acquired the nickname Dandogo, which means "idiot" in the language of the Limba people of northern Sierra Leone. By 1991, Momoh had been in power for six years and the nation was ripe for revolt.

Wounded in action

On Strasser's return from Liberia, he joined a unit fighting the rebel incursion in the east. The conditions for the government troops were wretched. Logistical support was poor, supplies of weapons and ammunition were limited and there was scant medical provision. On 1 May 1991, he received a shrapnel wound to the leg while defending a bridge.

“I was inside a bunker and I got blasted," he said. "It was a shell that actually landed on the sandbags." On another occasion when we spoke he said: "No casevac [casualty evacuation] procedures were made. In terms of helicopters or ambulances to shift the casualties . . . the problem was not with the level of training, but with the equipment that was available and the manpower. My disgruntlement stemmed from the fact that after I got wounded in action, I could not be evacuated, either by an ambulance or a helicopter."

Aware that they were fighting a war that their political masters would not resource properly, Strasser and other junior officers began plotting a coup. On 29 April 1992, they launched Operation Daybreak, raiding the office of the president in central Freetown as well as the lavish old presidential lodge off Spur Road in the West End of the city. They found President Momoh hiding in the bathroom of the lodge, wearing a dressing gown. He was bundled into an army helicopter and taken over the border to Guinea.

Strasser emerged as the public face of the uprising, in part because of his language skills – he spoke English well enough to read out a statement on the radio. As a captain, he was also of a higher rank than his co-conspirators. Some argue, too, that Strasser got the top post because those around him felt that he could be manipulated easily. "He was chosen in spite of, not because of, his leadership capabilities," says Joe Alie, a professor of history at Fourah Bay College in Freetown and the author of a 2007 history of the country since independence.

Joseph Opala, an American historian who first came to Sierra Leone in 1974 as a Peace Corps volunteer and has spent much of his adult life in the country, witnessed the wild early days of the new regime. Avuncular and bearded, he runs a project to restore the former British slave fortress on Bunce Island, near Freetown. Shortly after the 1992 coup, Opala was rounded up by soldiers and taken to State House, the white-walled seat of power in the city centre that bears an odd resemblance to a lighthouse.

The windows in the president's office had been shot out. Momoh's staff stood erect, in abject terror. Sitting around wearing camouflage fatigues and Ray-Ban sunglasses were the young officers who had mounted the insurrection. They were cleaning their Kalashnikovs and were stoned.

Strasser turned to Opala. "A wan know if America go recognise we gobment?" he said, speaking in Krio, the Sierra Leonean lingua franca. Krio is built on an English chassis but has a distinct grammatical structure and uses borrowed words from a plethora of other sources. In response to

Strasser's question ("I want to know if America will recognise our government?"), Opala asked him in turn if he had spoken to the American ambassador. The new leader replied that he had, but that he had not understood what the diplomat had told him. "En English too big," he said. "A no undastan natin way e talk."

An extraordinary scene ensued. At Strasser's direction, Opala left State House and walked through deserted streets to the US embassy, which at the time lay one block away. There he told a jumpy marine guard that he had a personal message for the ambassador from the coup leaders. He was allowed in and explained to the head of mission that the heads of the new government wanted to know if Washington would recognise it. The ambassador, a black American named Johnny Young, said that he had spoken at length to Strasser and had outlined the position of the US administration – that in general it did not acknowledge regimes installed by force but, in this instance, because the previous government had also not been democratically elected and considering the dire condition of the country, it was prepared to make an exception.

Ukrainian connection

In the early days, Captain Strasser's coup was popular. There were promises of a fresh start for the country. Young people mobilised to keep Freetown clean. Celebratory murals and other street art flourished. The new rulers of Sierra Leone called themselves the National Provisional Ruling Council (NPRC). Strasser was the council's chairman.

For all the jubilation, there was still a war to fight. Out in the bush, the army continued fighting the rebels. The junior officers who formed the NPRC had experienced the wretched conditions of the government troops. They wanted to improve matters, so besides tripling the size of the army, they went shopping.

There have been few better periods in history to buy guns than in the early 1990s. The Soviet Union had disintegrated, leaving huge arsenals in the hands of often unpaid and unsupervised officers. Dollars went a long way and official documentation was circumnavigable. Crucially, too, Sierra Leone's new leaders had a Ukrainian connection. During the cold war, the Soviet Union had funded scholarships for students from the developing world. Sierra Leoneans were among those who took up the chance to study in the USSR. One such was Steven Bio, who had studied in Kiev. A cousin of Julius Maada Bio, a member of the new junta, he had useful connections with gunrunners in Ukraine. He would be the go-between.

However, as the arms bazaar began to thrive abroad, the jubilation that had greeted Strasser's assumption of power at home began to diminish. In October 1992, the RUF took Koidu Town, capital of Kono District in the diamond-mining east. The capture of the town marked a step up in the conflict.

In Freetown, the NPRC government announced that it had uncovered an attempted coup and disarmed the instigators. Executions followed on a beach on the outskirts of the city, but the 29 people executed were considered to be innocent, and soon afterwards Strasser declared a nationwide period of mourning. "To people who were politically savvy, what it meant was there was no coherent government," Opala told me. "The conclusion was obvious – no one was in charge." (Nineteen years later, the mention of the executions stirred Strasser to anger. "Fuck off, man. In Texas they kill people every day," he said when I pressed him on the subject.)

Power in Sierra Leone was now in the hands of a group of very young men. "The children are running the country," it was said. A photograph of Strasser at the 1993 Commonwealth Heads of Government Meeting in Limassol, Cyprus, shows a young man in sunglasses and a T-shirt, emblazoned with the words "Sunny Days in Cyprus".

There were parties, too. Strasser made Valentine's Day a great national celebration, along with Bob Marley's birthday. The junta favoured pale-skinned women, creating a craze for bleaching among girls in Freetown. Women who tried to lighten their skin tone with chemicals were called "wonchee girls". Older Sierra Leoneans still mention that phrase readily when asked about their impressions of the NPRC. But perhaps the most telling indication of the onset of decadence in Strasser himself was his choice of accommodation.

Kabasa Lodge is in many ways the embodiment of all that is wrong with post-independence Sierra Leone. Built by the kleptocratic Siaka Stevens, it is a monumental structure the size of a missile silo or respectable late-medieval castle, and squats on a hilltop in Juba, in the West End of Freetown, with expansive views both out over the Atlantic and to the forested hills of the peninsula south of the city. It was here that Strasser chose to live.

The 1992 coup had decapitated the command structure of the army; brigadiers were expected to take their orders from captains and lieutenants. In the countryside, both rebels and the poorly trained soldiers were often more interested in looting property from civilians than in fighting each other. The line between the resistance and the rebellion became blurred, reflected in the neologism "sobel" – soldier by day, rebel by night.

By late 1993, though, the much-enlarged government army was close to defeating the rebels. In December Strasser called a ceasefire, but that turned out to be a mistake: the RUF regrouped and began setting up jungle bases around the country in 1994 and 1995. The rebels were a threat once more and the government was losing control.

Glittering prizes

In the south, the RUF attacked the facilities of Sierra Rutile, a company mining titanium ore, cutting off a crucial source of state revenue. The rebels set up a base in the town of Moyamba which put them within a day's striking distance of Freetown. Vehicle ambushes left few people willing to travel upcountry.

With the security situation deteriorating, the NPRC was becoming increasingly unpopular. It was then that Strasser turned to foreign fighters. White mercenaries are a charged subject in Africa, conjuring up a host of associations, from "Mad" Mike Hoare in the Congo of the 1960s to Richard Burton and Roger Moore in the 1978 film The Wild Geese and, more recently, the farce of the 2004 "wonga coup" in Equatorial Guinea. However, in Sierra Leone, shortly after South Africa's first multiracial elections in 1994, ex-apartheid enforcers re-engaged as soldiers of fortune and ended up saving huge numbers of lives. They nearly saved the country, too.

In February 1995, the NPRC engaged the services of a company called Gurkha Security Guards (GSG), which employed Nepalese ex-British-army troops led by an American, Robert MacKenzie. MacKenzie had fought in Vietnam and, in spite of an arm injury sustained there, he later passed selection for the Rhodesian SAS. He also worked as a correspondent for Soldier of Fortune magazine. His masterminding of GSG's involvement in Sierra Leone was a debacle: he was quickly ambushed along with Strasser's aide-de-camp, Abu Tarawalli. It is still not known for sure if those responsible were the rebels, or whether he was betrayed by Sierra Leonean army soldiers he was meant to be assisting.

After MacKenzie went missing, his wife asked Al J Venter – a writer with a long interest in mercenary affairs – to visit Sierra Leone to investigate what had happened. Venter discovered that a group of nuns had also been captured and taken to the camp where MacKenzie was held. The nuns were eventually released, but before then they saw the American strung up, and his heart cut out.

The next group of white mercenaries to land in Sierra Leone was Executive Outcomes, which blazed a trail for private military companies of the modern era. Composed predominantly of former South African special forces troops, Executive Outcomes was active in Angola during the civil war there, fighting both for and against Jonas Savimbi's South African-funded rebel army, Unita.

The brokers of the deal that brought Executive Outcomes to Sierra Leone included Simon Mann, later of the botched "wonga coup"; Tony Buckingham, who now runs Heritage Oil, a company whose prospectus hints at the risk that the media may mention his previous mercenary adventures; and Eeben Barlow, a former South African special forces officer. The role of Executive Outcomes was to combat the rebels. The mercenaries would be paid in diamond concessions and cash.

They arrived in Sierra Leone in small numbers – about a hundred on the ground at any one time. Most of the operatives were black but theleadership was white. They used helicopters, they had their own logistical train and they were fearsomely competent. "These people knew Africa," Venter said. "They set up their own supply units . . . they brought everything with them. They drove [the rebels] well away from Freetown, then they launched an operation into Kono. They did it; they turned the war around in record time."

Joseph Opala recalled how Executive Outcomes would give a radio to each of the paramount chiefs, the leaders originally appointed from the ranks of local kings and queens by British colonial administrators at the end of the 19th century. "They said: 'If you call us we will be there in 15 minutes.' And they were."

The mercenaries achieved what thousands of UN peacekeepers five years later were unable to do: they stopped the war. "At a total cost of $35m [just one-third of the government's annual defence budget], the fighting in Sierra Leone had ceased and over one million displaced persons returned to their homes," wrote P W Singer of the Brookings Institution in his book Corporate Warriors: the Rise of the Privatised Military Industry.

“They did what they were here to do – that I can assure you," Strasser told me. "In fact, fighting stopped. It was a war machine that was capable of handling the security difficulties there at the time."

But the mercenaries were soon forced out of Sierra Leone by other countries' disapproval. There was substantial international support for a peace accord that was negotiated in Abidjan, Côte d'Ivoire, in 1996, and the RUF made withdrawal of foreign forces a provision of signing it. Executive Outcomes left in January 1997. Without a disarmament programme in place, the Abidjan agreement proved ineffective. Clashes continued and after another military coup in May 1997 the violence escalated once more. In January 1999, the war reached its nadir when RUF fighters sacked Freetown in Operation No Living Thing.

As for Strasser, he was deposed in a palace coup on 16 January 1996. He had gone to inspect a passing-out parade at the military trainin academy in Benguema, less than 30 miles from Freetown. In the afternoon he went, without a substantial security escort, to a meeting at the defence headquarters at Cockerill, back in the capital city. There he was overpowered and bundled into a helicopter and flown to Guinea, just as had happened to Joseph Momoh four years earlier. Strasser's successor, the leader of this second coup, was Julius Maada Bio. The new leader was still only in his early thirties.

When I asked Strasser why his reign ended as it did, he refused to accept there had been a coup. He claimed he had merely stepped down at the end of the ten years of military service for which he had signed up. That statement is fantastical, and must be discounted.

Anything for a quiet life

The post-deposition period is perhaps the strangest in Strasser's unusual life, taking him from West Africa to Coventry in the West Midlands. When the international community had negotiated with the NPRC over the reintroduction of civilian rule, one of the incentives offered to members of the junta in return for relinquishing power was the opportunity to study in the west. And even though Strasser had eventually lost power by less graceful means, he was able to take up this chance.

Warwick University's decision to consider admitting him was controversial. "When it became known who he was, there was a lot of disquiet in the law school and the university," recalls Roger Leng, an expert in criminal law at Warwick who later taught Strasser. There was a fierce internal row over whether he should be allowed to enter as a student, despite assurances from reputable sources to the university that Strasser was not responsible for human rights violations.

Eventually he was accepted and took a foundation course to compensate for his lack of formal qualifications. The intention was that he would then progress to a law degree.

Leng was surprised when he met Strasser for the first time. "He was quiet. I don't think really he was equipped to study at this level," he said. "I'd expected a swaggering, arrogant guy and he was quite the opposite."

Strasser's second life as a civilian in England did not go well. His unwanted celebrity was a problem. He took up residence in an anonymous red-brick terraced house at 47 Poplar Road in suburban Earlsdon in Coventry, the city nearest the university, but the local and national press began to take an interest in him. He claims, too, that his stipend was inadequate. It even turned out that among Strasser's fellow students in 1996 was a niece of one of the victims of the extrajudicial killings of December 1992.

According to him, the woman spoke against him on television and lobbied against him. The archives of the Boar, Warwick University's student newspaper, mention inquiries launched into his presence. "The university's belief that Strasser's studies will contribute to the democratisation process has been attacked by those who consider that an individual with such a brutal background should not be afforded acceptance within wider society," the Boar reported in October 1996.

Later he had an unsuccessful affair with a supermarket checkout girl. "She knew who I was, because the papers in Coventry had things about me," Strasser said. "She knew I was a former dictator."

Warwick University closed its file on Strasser in January 1998. A spokesman for the university, Peter Dunn, believes he left campus before then. "My recollection was that he wrote to the university staff saying that he was leaving," Dunn said. "One of his concerns was that he was fed up with his history in Sierra Leone being constantly brought up." Strasser corroborated that account. "I saw front-page articles saying 'former dictator' and 'human rights violations'," he said. "It was impossible."

After dropping out of Warwick he moved to London, but there he found no peace. Albert Mahoi, a Sierra Leonean who goes by the nickname of Carlos, was running a business in south-east London that offered cosmetics, money transfers and international calls when he met Strasser. Mahoi recalled encountering him at a nightclub in Camberwell; another Sierra Leonean exile was abusing him and Mahoi felt he had to intervene.

“I said: 'Don't do that – he was our president,'" Mahoi told me. "I talked to Strasser, I told him to calm down." He bought the former head of state a bottle of Courvoisier. "He was stressed up; you know when someone loses everything. There was no respect for him."

With the Guardian newspaper questioning why a one-time West African strongman was living in London, Strasser left the country. The Home Office would not comment on whether his visa had been revoked. In December 2000, he went briefly to the Gambia and then back to Sierra Leone. And he is still there.

Moving with the times

The civil war finally ended in 2002 after a Blair-led British military intervention stiffened a floundering UN peacekeeping mission. The peace has held, and in November the country will hold its third multiparty election since the war's end. Large iron-ore mining projects are coming on line, and the IMF predicts massive GDP growth of 51.4 per cent this year.

Yet Sierra Leone remains impoverished; it ranks 180th (out of 187 countries) in the UN's Human Development Index and per-capita GDP stands at just $325 a year. The country also has a large pool of marginalised ex-combatants and other young men who continue to pose a threat to stability. Despite enormous expenditure of foreign aid, corruption remains endemic and progress on infrastructure frustratingly slow.

Desmond Luke is a former chief justice who trained at both Cambridge and Oxford. "One of my biggest sadnesses is when I travel out of Sierra Leone and I come back," he told me recently at his house in Freetown. "The only change one really does see is it seems to get dirtier."

Some of the figures from the war years are still in politics, too. Maada Bio, who deposed Strasser and was briefly head of state, is now the candidate for the main opposition party in the November presidential election.

Strasser lives quietly with his mother, Beatrice, in the house he built at Grafton, east of Freetown. The once-elegant white villa is run-down and the walls are stained. Across the potholed road stand the burnt-out ruins of another house that Strasser had built while in office, but which was bombed by Nigerian fighter jets during the civil war.

He receives a government pension of 200,000 leones (£30) a month. That is a recent improvement on the 64,000 leones (£9.40) he used to get. He is desperately poor and does not even have a mobile phone to hand as he sits by the roadside in the afternoons. "It's a new set of circumstances and I've got to accept them," he said of his life with his mother.

I asked Sheka Tarawalie, Sierra Leone's deputy minister of information, why the former leader receives such meagre support. "You know, Strasser was not an elected head of state," Tarawalie said. "That is one of the problems. He came in as a military man."

"Bad dictators"

One evening last summer, at the start of the rainy season, I arranged to meet Strasser for a final dinner. I went to see him with a friend and a British researcher resident in Freetown. We drove over the mountain road and picked Strasser up from his house.

He sat in the front seat of my Land Rover, wearing trainers and cut-off jeans. At his suggestion, we went to eat at a Safecon petrol station on the main road upcountry. There we sat at a table outside in the evening light.

It did not go well. He was drunk at the start of the meal and became agitated. When I raised his time at Warwick, he raged at me – I was his assassin, he said. I was the president of America. He became increasingly unstable and threatened to have us arrested, only to change his tone. "I'm not going to arrest you," he shouted. "Otherwise you'll say I'm Idi Amin or another bad dictator like Colonel Gaddafi."

Then he wrote this, in block capitals, in my notebook: "Europe still continues to underdevelop Africa. Africa's raw materials are Europe's tool to keep black Africa under so that western Europe continues to improve. Answer, 3,500 words."

There was something of Lear in Strasser that evening, the broken king raging at the injustices of the world. I met him again several times after that and he was always sober and lucid. Yet that night I had seen a different Valentine Strasser and begun to understand something of the burdens he carried. As we drove back over the hills in the tropical dark, it was clear to me what a terrible misfortune it was for him to have been crowned by accident.

James Appleton contributed additional reporting from Warwick University