Differences on Syria crisis loom over G20 summit
The G20 was created in the wake of the 2008 financial crisis to encourage global economic collaboration.
But this year, instead of being united by common economic interests, the participants risk being divided by the chasm which has opened up over how to respond to what is happening in Syria.
On the eve of the summit, the de facto leaders of the two opposing camps - the US and Russian presidents - set out their positions.
Barack Obama said he was convinced that the chemical attack in Damascus last month was the work of the Syrian government and - with or without UN backing - the world was obliged to react: both to prevent further attacks and uphold the Chemical Weapons Convention, and also to prevent the entire system of "international norms" and rules from being further flouted.
Vladimir Putin dismissed as "absolutely absurd" the idea that Syrian President Bashar al-Assad would risk retaliation by using chemical weapons.
The Russian leader added that unless convincing evidence, gathered independently and endorsed by the UN, was forthcoming, any claims against the Assad regime made by the US remained groundless.
His foreign ministry warned that any US military retaliation without UN approval, far from upholding international norms as Mr Obama argued, would be a "gross violation" of them and "have all the attributes of aggression" - a very serious charge for one nation to make to another.
Syria is not officially on the G20 agenda, so any discussion will be informal. Nonetheless, how the different countries line up behind these two positions will be illuminating and could have some bearing on how this crisis will play out.
Joining Russia in opposing US action will no doubt be China, given it too has consistently vetoed attempts to impose pressure on the Assad government at the UN Security Council and repeatedly insisted that any solution must be political. India and Indonesia's views are less easy to identify.
But South Africa has spoken out clearly against the idea of military action without UN approval, as has Argentina's President Christine Kirchner.
And her two Latin American colleagues, the presidents of Brazil and Mexico, may be in no mood to support Mr Obama, given their publically voiced annoyance at reports which surfaced this week, based on Edward Snowden's secret National Security Agency material that suggested they might have been targets of US eavesdropping operations.
For his part, Mr Obama knows he has the backing of French President Francois Hollande for military action, but - because of last week's vote in the UK parliament - only diplomatic, but not military support from British Prime Minister David Cameron. Turkey has long advocated intervention in Syria, and Saudi Arabia is part of the Gulf coalition active in backing Syrian rebels.
Other Western allies at the table include Canada, Australia, South Korea and Japan, as well as Germany and EU leaders. But their separate views on the difficult question of whether or not to strike back against the Syrian military without UN approval are likely to be nuanced. Italy, also at the G20 table, has already voiced its objections.
What is more, also at the G20 table will be the UN Secretary General, Ban Ki-moon, who earlier this week stated clearly that the use of force against another nation could only be legally justified when in self-defence or with UN Security Council approval.
Up against that, Mr Obama may need all his rhetorical skills to win over any new converts to his conviction that this is a moment of global moral responsibility and potentially historic significance, which ought to trump an international legal framework which has shown itself over the past few years to be paralysed by indecision and disagreement.