For Mr Surayud, his departure is part of a self-imposed restriction set by Thailand's military junta. Elections in December will bring a new civilian government to power.

Since its inception in 1989, the forum on Asia-Pacific Economic Co-operation has always represented style over substance with its loose, consensus-based statements. With image to the fore, the symbolism of so many top leaders being on the way out is sure to hang heavily. Outgoing leaders means changes in relationships and policies across a region already facing shifting strategic assumptions.

Officially, the 21 leaders from across the Pacific Rim, including mainland China, Russia, Canada and even Peru, have a busy agenda during their three-day session starting on Friday. Their economies, after all, represent some 60 per cent of the world's gross domestic product and about half of all global trade.

Climate change, energy and trade security, as well as food safety, are expected to feature heavily. More specifically, progress is expected on specific goals and action plans to improve energy efficiency. Leaders will also be under pressure to show fresh resolve to finalise negotiations in the long-delayed Doha round of trade talks.

For all that, it is the unofficial programme that is arguably of most value - the side meetings between individual leaders, many of whom rarely find themselves in the same room together, much less talking.

President Hu Jintao , for example, is expected to sit down separately with Mr Bush, Mr Abe, Mr Roh and Mr Howard. Mr Abe, meanwhile, is keen to pursue his 'arc of freedom' in side sessions with other key democratic leaders, particularly Mr Howard and Mr Bush.

In this regard, the departure of so many senior leaders adds an extra frisson. 'There is a spark of change in the air,' said one veteran Asian envoy. 'Behind the scenes there is going to be a lot more manoeuvring and positioning than usual during the side meetings.

'With so many changes ahead, officials will be casting their eyes around to see who will be useful in future and what legacies and policies will remain.'

The mood is not lost on Mr Bush. Even as he enters his own twilight - US voters are due to elect a new president in November next year - he is still preparing the ground to cope with a new counterpart from Australia, a key Washington ally.

Combining the Apec work with a formal state visit to Australia to honour Mr Howard, he is still making sure he has enough time to meet his host's opponent in elections slated for November or December.

Despite Mr Howard's considerable reputation as a political survivor, former diplomat Kevin Rudd has built a solid lead in the polls for both himself and his Labor Party. Buoyed by years of strong economic growth, Australians are showing signs of ditching habitual caution at the ballot box in favour of the untested Mr Rudd.

The US ambassador to Australia, Robert McCallum, dashed off an invitation on behalf of Mr Bush this week and Mr Rudd did not waste any time in accepting, apparently. 'I indicated that I would of course be delighted to meet with President Bush and his team,' Mr Rudd told The Australian newspaper.

Under his leadership, Labor is expected to seek a deeper, more nuanced relationship with Beijing and the rest of Asia, while maintaining the traditional alliance with the US. One of the key strategic tests in that alliance will come within a year of an expected Rudd victory - Labor has pledged to withdraw Australian troops from Iraq.

With a troubled legacy on their minds, the Bush team is keen to make its presence felt amid nagging questions about the attention Washington is giving the region. The United States' delegation will be more than twice as large as any other, with more than 1,000 US officials, business executives and support staff.

Mr Abe's predicament is far more difficult to read. His ruling Liberal Democratic Party lost control of the upper house in elections in July. And despite the historic nature of the defeat by the opposition Democratic Party of Japan, Mr Abe is bucking convention by staying on instead of resigning.

This week highlighted the pressures on the 52-year-old who has yet to complete a year in office. Seeking to ease internal party criticism, Mr Abe embarked on a sweeping reshuffle of his cabinet, replacing 11 ministers and keeping five.

Japanese political analysts remain unsure whether the re-shuffle will be enough to save Mr Abe from being forced to step down amid ongoing debates within the LDP. Those pressures are expected to intensify once a new parliamentary session opens soon after his return from Apec.

Mr Abe, of course, has worked hard to rebuild ties with Beijing, given the difficulties during the rule of his predecessor, Junichiro Koizumi. He may have pushed a more assertive nationalism, but Mr Abe has also recognised deepening economic and cultural ties with the mainland, now Japan's biggest trading partner.

'Our only message will be business as usual,' said one Japanese official. 'Mr Abe will be making clear that his key policies remain ... and that puts a priority on the need to develop relations with China on all fronts.'

The legacy of South Korea's Mr Roh will also face close scrutiny amid a touch of uncertainty over what will follow.

Mr Roh's single five-year term ends with elections in December - a rule marked by friction with Washington, Seoul's traditional ally, and a burgeoning relationship with Beijing.

Mr Bush's tough line on North Korea stood in marked contrast to Mr Roh's efforts at engagement - with both approaches facing criticism after Pyongyang's first nuclear weapons test last October.

The favourite to replace him is opposition Grand National Party candidate Lee Myung-bak, the popular former mayor of Seoul. Mr Lee is promising stronger economic policies and efforts to strengthen the traditional alliance with the US, which still bases troops in South Korea. He is also likely to be more cautious in engaging the North.

Mr Roh is expected to brief regional leaders on his upcoming summit meeting in early October with North Korean leader Kim Jong-il - only the second such meeting with the head of the world's last Stalinist regime.

The meeting is expected to be heavy on symbolism but it remains to be seen whether it will result in specific co-operation between the neighbours, who are still technically at war.

It comes amid signs of renewed progress in the international effort to rid North Korea of its nuclear weapons - moves which could lead to diplomatic normalisation.

Mr Kim, of course, is one regional leader who will be conspicuous by his absence in Sydney. The hermit state has never expressed an interest in joining Apec. As he keeps an eye on proceedings from Pyongyang, he can be safe in the knowledge that his rule will survive at least some of his regional rivals.