The potential costs are all too clear. A nuclear-armed North Korea threatens to destabilise an already-tense East Asian region, risking an arms race.

The diplomatic fallout is already visible. The future of the six-nation talks geared at disarming North Korea is at risk after their failure to produce progress in Beijing last week. The South's once-vaunted 'sunshine policy' towards its enemy also lies in tatters.

Mr Ban long advocated both. He even touted the six-party negotiations involving both Koreas, the US, China, Japan and Russia as a potential security body to ease regional tensions in the long term.

A role of great responsibility and pressure but little direct control, the secretary-generalship is the job of Mr Ban's life. He is the first Asian to win the post since Burma's U Thant 45 years ago.

It has been a rise containing a touch of the fairytale. The eldest of six children born in a farm village in Japanese-occupied Korea during the second world war, Mr Ban learned the virtue of hard work during years of struggle. Devoting himself to study as a schoolboy, he worked particularly hard at English.

The UN-brokered end to the Korean war fed his interest in international affairs. In 1962 he met then-US president John F. Kennedy after winning a study trip to the US. When Mr Kennedy asked him about his career ambitions, Mr Ban's mind was already made up: 'Diplomat,' he replied.

Since then it has been a life marked largely by hard work and quiet backroom achievement at South Korea's Foreign Ministry, which he joined in 1970.

As well as Seoul, he served in Washington, Austria and New York. UN issues were high in his thoughts as he climbed the diplomatic ladder, becoming director of the ministry's UN division in 1980.

He forged a wider international profile in the days after September 11, 2001, as he worked directly under Mr Annan on secondment to the UN secretariat, co-ordinating the international response to the terrorist threat.

As his campaign for the job intensified over the past 10 months, Mr Ban put the Korean issue at the forefront, while successfully appealing to both China and the US for crucial backroom support.

'Look at the recent history of the Korean Peninsula and you will see that [South] Korea is very much a child of the United Nations,' he said in an interview earlier this year.

'We think we are a model of many member states of the UN, one that has been able to realise all the important goals and ideals of the UN charter,' he said, citing South Korea's current peace and prosperity, as well as its hard-won development of human rights and democracy. 'We are proud of what we have achieved and now Korea wants to play a greater role in the world community.'

His campaign also highlighted his desire for a gradual, measured reunification with the North - which has technically been at war with the South since they were split at the end of fighting in 1953. The prospect of a sudden political and economic collapse in North Korea may set some conservative security figures drooling, but South Korea has long feared the impact of anarchy. Seoul, they believe, would be left cleaning up the mess - a fear that has seen it moderate the international condemnation of October's nuclear test.

Mr Ban has spoken repeatedly of the need for efforts to lessen the economic gap between the two Koreas to soften any future transition, as well as the prospect of a Korean 'commonwealth' before final reunification is achieved.

'We like first of all to breach the gap between South and North in terms of economic capacity, social aspects, as well as political ideology,' he said earlier this year of gaps that have appeared even wider since the test.

Battling rival contenders for final approval by the UN Security Council and the General Assembly in October, Mr Ban had to repeatedly defend himself against a perceived lack of charisma and strength, given his modest image. His campaign, after all, had highlighted his more humble qualities as he spoke of his abilities as a 'harmoniser and bridge builder'. Many UN insiders questioned whether he could stand up to Washington, a traditional South Korean ally but one wary of his predecessors.

He acknowledged such questions in his acceptance speech, saying: 'Modesty is about demeanour, not about vision and goals. It does not mean a lack of commitment or leadership.'

Mr Ban's life appears to leave very little time for play. A father of two adult daughters, he recently expressed regret to The New York Times about his lack of sporting interests or hobbies. Golf was his only game, he said, but he had only played 10 times in the past four years.

Mr Kim's pastimes, however, are even more sedate. No intelligence dossier on the North Korean's shadowy life is complete without descriptions of his love of rich food - he has the finest caviar and best cuts of sushi regularly flown in - and fine wines. Receipts from Hennessy have estimated Mr Kim's annual expenditure on cognac alone at more than US$800,000 per year, although some reports suggest he has toned down his excesses due to health concerns.

His life of luxury has been highlighted recently as the US and Japan sought to enforce sanctions passed by the UN Security Council in the weeks after the test. Lead glass, large screen LCD televisions, wine and liquor were on the list of banned imports, as well as jet skis and high performance cars. Defectors have long relayed tales of his high-flying parties, his love of the latest technology, and his extensive library of Hollywood and Asian movies - all far beyond the reach of the North Korean masses, most of whom are rural peasants.

The secretive nature of the North Korean communist regime and its propaganda machine mean hard facts about Mr Kim are sketchy at best. His official biographies are of little use, suggesting his birth at Mount Paektu in the far north in 1942 was marked by a new star and a double rainbow. Other records suggest it was more likely in Siberia in 1941, while his father, later to become North Korea's 'Great Leader', Kim Il-sung, served as a Korean exile in the Soviet military.

Educated in Beijing, Pyongyang and Malta, Kim Jong-il began his relatively charmed rise up the ranks of the Korean Workers' Party in 1964 - not long after Mr Ban's trip to the US when he met JFK.

By 1974, Mr Kim was heading the party's crucial organisation and propaganda wings and was his father's designated successor. He eventually took power after Kim Il-sung's death in 1994 and became known as 'Dear Leader', continuing the Kim family's personality cult. It was to be the only dynastic succession in Marxist history.

In the wake of the tests, it is not surprising that succession issues are once again surfacing among seasoned North Korean watchers. They like to call their craft 'Pyongyangology'. Mr Kim, they note, is now older than his father when he chose him as heir apparent, but no clear successor has emerged among his three sons.

A South Korean intelligence report last week suggested a possible end to the dynasty with the death of Mr Kim. Top military officials would probably take over in some form. The current favourite is O Kuk-ryol, a 75-year-old general close to Mr Kim and a long-serving member of the ruling Central Military Commission.

For some, the tests appear to confirm the sense of paranoia faced by an ageing Mr Kim. He has proved recalcitrant towards his only significant ally and patron, Beijing, and has been gripped for years by fears of a full-blown US invasion. Both western and Asian military analysts acknowledge that such a threat is reduced by the build-up of a nuclear arsenal. As the New Year begins, a second test is a distinct possibility.