The Group of 20 leading economies proved its mettle in 2009 when leaders united to avert the depression that threatened. Four years on, that spirit of unity is broken, and policy co-ordination is proceeding haltingly, if at all. Today’s challenges are different, but crisis – political and economic – is again in the air. The world needs the G20 leaders to rekindle their willingness to work together when they meet in St Petersburg this week.

The prospect of an imminent US bombing of Syria hangs over the summit. But it is also a diplomatic opportunity. President Barack Obama must make the most from face-to-face encounters with the leaders of Russia, China, and Saudi Arabia. These states in turn must fulfil their duty to help rebottle the genie unleashed by the gassing of Syrian citizens.

US action will now be decided on Capitol Hill. But Mr Obama’s ability to achieve an understanding with other leaders matters for the aftermath of any strike – on the ground in Syria and in terms of the diplomatic fallout. It may even sway votes back in Washington.