Leading article: Superpowers in search of the next world order

One advantage of the medium-sized power status to which the UK is becoming reconciled is that London has the luxury of observing from afar as the really big beasts prowl around the global compound, sizing each other up. This week's visit to the United States by Xi Jinping of China is a classic case in point.

Mr Xi might appear, in terms of ranking and schedule, to be a visitor of secondary importance. Vice-President is not, by itself, a position of huge significance, and Mr Xi will take a well-trodden route – from Washington to Iowa farming country to Los Angeles – that is as much about VIP tourism as politics. But this is not how the US sees this trip, nor how China sees it, for that matter.

Mr Xi is not just any Vice-President. He is also the second most important official of the Chinese Communist Party (after the General Secretary and President, Hu Jintao), and one of three vice-chairmen of the Central Military Commission. This is a combination that marks him out as the man most likely to succeed President Hu when he retires, as he is expected to, as head of the Communist Party in the autumn, with accession to the presidency to follow a year later.

This is why his US visit was preceded by a flurry of commentary in the American media that veered from the fawning, via the curious, to the alarmist, and why China's official Communist Party newspaper, the People's Daily, also considered it worth an extended commentary. This is also why Mr Xi was received yesterday by President Obama in the Oval Office, why he was wined and dined the previous evening by diplomatic luminaries, including Henry Kissinger, the architect of the late Richard Nixon's historic opening to China, and why he will also be received at the Pentagon.

Both for the short term and the longer term, these are high-stakes encounters. In the short term, Mr Xi may be President Hu's designated successor, but one mis-step, particularly abroad, would immediately cast doubt on his suitability. A US trip is the ultimate rite of passage for an aspiring 21st-century Chinese leader. Mr Obama, for his part, is embarking on an election campaign in which China's rise to rival superpower status could loom large, and toughness in defence of the national interest will be called for. Any hint of softness will be seized upon by the Republican candidate (whoever it turns out to be) and used relentlessly against him.

But Mr Xi's five-day stay is also seen in both countries as potentially setting the tone for the longer term. If Mr Xi passes his American test and Mr Obama secures re-election, the impressions each man takes away this week stand to colour the personal relationship and that of these two countries for the best part of four years – crucial years in which the speed and direction of China's development should become clearer, as will the sustainability of US power.

The view from the sidelines is one of two great forces warily taking each other's measure: China moving clumsily and unpredictably, chafing at existing global rules, and the US moving cautiously and deliberately to redeploy its forces in anticipation of a new world order. The changes in US defence structures announced by Mr Obama last November were directed towards one end, and one end only: towards maximising the US position in the event that China becomes a more assertive, even aggressive, Pacific power sooner than at present envisaged.

The priority of the Chinese guest and his American hosts this week is to broach some of the existing tensions – China's undervalued currency, trade imbalances and human rights, and America's new defence posture, to name but a few – while laying positive foundations for the future. But this is a risky endeavour that requires mutual restraint and continuing US vigilance, and could all too easily come unstuck.