The London summit of 1933 marked the moment at which co-operative efforts to manage the Great Depression collapsed. The summit of the Group of 20 countries, in the same city, on April 2, must turn out quite differently. That may seem a simple task. It is not. The usual platitudinous communiqué would be a catastrophe.

The world now needs change it can believe in. Only Barack Obama, the US president, can provide the desired leadership: he is untainted, popular and leader of the country that, for good and ill, remains central.