This evening sees the ceremonial prelude to the business of the G20 summit, with a reception at Buckingham Palace and dinner at Downing Street. It will take the buzz of helicopters, the sleek convoys of official cars, the flags, the road closures and the sporadic clamour of 1 April protesters to complete the scene. But for 24 hours or so, London will be the centre of the world.
Nineteen national leaders and the President of the European Commission will be assembled at the same place at the same time. Security will be inconvenient – perhaps hellishly so, to the rest of us. But these visitors, unlike many a high-level guest before them, are not being hidden away. They will see something of London – and, perhaps, London will see something of them.
The summitry itself will be brief, heavily scripted, but also, until the final sentence of the final press conference, attended by that tiny fraction of uncertainty about the outcome. No leaked versions of communiqués can ever substitute for the real thing. As with all such gatherings, however, what happens on the margins will be at least as significant as the central event. And here, there is no doubt about who will be the star of the show. All eyes will be on Barack Obama, making his debut as US President on the international stage.
A US president is guaranteed to be the focus of attention almost anywhere he travels in the world. The entourage and the paraphernalia of the office by themselves say that this is the most powerful individual and the most powerful institution in the world. As a presidential candidate, though, Mr Obama had a special aura and aroused special hopes, not only in comparison with the man he aspired to succeed, but for himself – through his presence and his oratory – and for what, as the first black president, he would stand for.
His election instantly rehabilitated the United States throughout the world. It showed America at its idealistic best and transformed the international atmosphere overnight. His inauguration was suffused with a level of enthusiasm and good will, at home and abroad, not seen or felt since the election of John F Kennedy. His first foreign policy moves only reinforced the impression of an America purged and renewed. Guantanamo was to be closed; torture was denounced; a timetable set for the promised withdrawal from Iraq; policy towards Afghanistan was comprehensively revised. A hand was extended to Iran, then to Syria. The US pledged to "press the reset button" on fractious relations with Russia.
The international spell cast by Barack Obama still holds. His welcome in London – if anyone can get near enough to catch a glimpse of him – will be rapturous. Many sense the chance to usher in what could become a golden age in relations between Europe and the United States. It is early days. Such optimism may prove misplaced. That there is a such a source of hope, though, offers welcome consolation in these otherwise dark times.
It will be up to Mr Obama, and his advisers and speechwriters, how much of the magic lives on through tomorrow's summit and afterwards. His extensive tour – to the French-German border for the 60th anniversary of Nato; to the Czech Republic – the "new Europe", so beloved of George Bush – and to Turkey, fulfilling his promise to visit a Muslim country early in his presidency – amounts to a test, and a severe one, of his intentions, his authority and his diplomatic touch.
For impatient Americans, some of the Obama magic is already wearing off. His promises of Congressional bipartisanship have so far come to naught. His approach to the financial crisis, while rhetorically more in tune with popular feeling than anything said by any national leader, has drawn criticism. Many offices are as yet unstaffed; some of his early choices turned out to be misguided. The US audience has yet to be convinced that their new President is equal to what they see as the vast scale of the task in hand. For all that she is now in his team, Hillary Clinton's campaign jibe about the Oval office being no place for on-the-job training can be heard again on the lips of others.
Few US presidents have been made or broken at home by a foreign tour. And it is often not until their second term that a president, with an eye on his legacy, looks beyond his own shores. The most that this trip will do for Mr Obama at home is to add a little lustre – or not. Even if he does not yet seek to make his mark in the world, however, the world is crying out for him to do so. Less than three months in office, the chief executive of the world's most powerful economy is presented with a lifetime's opportunity to lead. How he responds will show how much of a foreign policy president he wants to be.