The jokes about Barack Obama as saviour and messiah are wearing a little thin now. But yesterday he signalled that even if he cannot save the world he is deadly serious about changing it. By telling a town-hall-style meeting in Strasbourg that he wants "a world without nuclear weapons" - a theme he will develop in a major address on proliferation to be delivered in Prague tomorrow - he stunned much of the usually sceptical diplomatic fraternity. Not because the statement was entirely new: Obama said the same thing as a candidate, including before a vast crowd in Berlin last July. But candidates say lots of things. Few believed that Obama, once elected, would seriously try to pursue such an idealistic, even utopian ambition. Yet on this first trip overseas, and despite all the multiple pressures on him at home, the new president has given every indication he means business.  The breakthrough he made in his first face-to-face encounter with his Russian counterpart, Dmitry Medvedev, in London on Wednesday was precisely in this area - with the two leaders agreeing in principle to slash their nuclear arsenals, probably by a third. Obama advisers explain that the US and others cannot plausibly pressure Iran to give up its nuclear ambitions if they do not honour the nuclear non-proliferation treaty - which calls on all existing nuclear powers to work towards disarmament. Until now any acknowledgement of that obligation was mere lip-service. But Obama's words and deeds over the last 72 hours suggest the US approach to that treaty has now changed. And that is far from the only shift. Every aspect of Obama's conduct on this European tour marks a radical departure from the immediate past. While George Bush's administration usually appeared to be laying down the law to its supposed partners - "You're either with us or with the terrorists" - Obama has stressed that he is keen, as he put it yesterday, to "listen and learn from our friends and allies". His entire approach to the G20 was a break from the go-it-alone unilateralism associated most intensely with Bush's first term. Obama spoke and acted as a multilateralist. So he admitted that the US had not got everything its own way at the London summit, adding that that was how it should be in the 21st century. "If there's just Roosevelt and Churchill sitting in a room with a brandy, that's an easier negotiation," the president said. "But that's not the world we live in, and it shouldn't be the world that we live in." Instead he referred to the US as a "peer" of its fellow G20 members, bowing to French and German demands for tighter regulation of international finance. On that last point, he reportedly acted as a conciliator, cooling down a simmering dispute over tax havens between the presidents of France and China, taking each of them to one side, in the corner of the negotiating room.  It is hard to imagine that role - of US as honest broker and umpire - being played by Obama's predecessor. After eight years in which Washington seemed disdainful of the rest of the world, dismissing France and Germany as "old Europe", Obama's warmth in both countries yesterday represented another sharp shift.  Six years ago, the US Congress ordered French fries to be renamed "freedom fries". Yesterday Obama was mobbed on the streets of Strasbourg and engaged in a mutual love-in with Nicolas Sarkozy. The president seems to have been at pains to show both humility, admitting past US "arrogance", and respect for the other nations of the world.  At his London press conference, he went out of his way to call on "foreign" journalists, before hastily correcting himself with a smile. "Actually, I'm the foreigner here," he said.