Following its gathering in St Petersburg, the Group of 20 leading nations is more deeply divided over Syria than ever before. The US and France, with the support of other western countries, are in favour of air strikes which Russia, backed by China and fellow emerging nations, staunchly opposes.
However, the disagreement over Syria is not an unbridgeable divide between former Cold War foes. Despite Russia's robust rhetoric, its position is shifting. The Kremlin recognises publicly that the chemical weapons attack outside Damascus on August 21 has changed the situation fundamentally.
Like the US and the rest of the West, Russia has signed and ratified the 1925 Geneva protocol banning the use of chemical weapons following the horrors of the First World War. Thus far an unwavering ally of Bashar Al Assad, Vladimir Putin has signalled that he is prepared to confront the Syrian leader over the gas attack.
There are three reasons for Moscow's new stance. First of all, the use of chemical weapons is unacceptable for Moscow because it would set a dangerous precedent for rogue regimes like North Korea. If the UN inspectors produce evidence beyond reasonable doubt that the Assad regime - not the rebels - was behind the lethal attack, the Kremlin would probably agree to a UN Security Council resolution condemning the brutal killing of civilians.
Moscow is prepared to go much further. On the margins of the Krynica Forum in southern Poland, which I attended last week, Russian officials and experts indicated that the Kremlin would consider suspending Russian arms deliveries and forcing the regime to give up its entire chemical weapons arsenal.
One welcome side effect would be to stop such weapons from falling into the hands of Al Qaeda-affiliated extremists - a growing threat about which Russia has consistently warned. In the past, Moscow got tough with Iran, for example, in 2010 when it agreed to strong sanctions on Tehran for its nuclear programme. This suggests that Russia is willing to act on Syria as long as there is independent evidence for the Assad regime's involvement.
To take action in the Iranian but not in the Syrian case of violating international norms would expose Moscow to the same accusation of hypocrisy and double standards it regularly levels against Washington.
The second reason for Russia's position is the lasting legacy of Iraq and the diplomatic disaster for the Russian leadership when it excluded itself from the post-conflict settlement. It is in the country's national interest to try to shape the long-term future of both Syria and the wider Middle East.
If the Syrian regime is found to have used internationally prohibited weapons, holding the UN Security Council hostage will damage both Russia's international standing and its interests in the region. That, in turn, will undermine the Kremlin's attempt to project power worldwide.
The third reason for Russia's position is the prospective thaw in relations between Iran and the West since the election of President Hassan Rouhani. Damascus and Tehran may be Russia's last allies in the wider Middle East, but close ties with them give Moscow some considerable power and influence. Both regimes are key to a permanent political settlement not just in Syria itself but also in the conflicts involving Israel, Palestine and Egypt.
In addition to deterring chemical weapons attacks, the US-led coalition against Mr Al Assad that includes Saudi Arabia and the other Arab Gulf states should make use of Moscow's links with the Syrian and Iranian elites for a diplomatic offensive to address the worsening humanitarian crisis and work towards a political solution.
So the US-Russian clash over Syria boils down to the issue of military intervention. The UN doctrine "responsibility to protect" offers the option to intervene in an internal conflict to defend civilian populations from genocide and other crimes against humanity. But it is no licence to launch military strikes without a UN mandate - or at least, possibly, a majority in the General Assembly if the Security Council remains deadlocked.
Unlike in Iraq, Barack Obama is planning a military intervention that is temporary and limited in scope. However, the Russian counter-argument is that surgical strikes tend to produce unwinnable wars, not perpetual peace. Aerial bombings also have uncontrollable consequences for the civilian population and provide support for extremists on the ground, as happened in Libya. Outside interventions favour regime change, which is illegal - though Russia and China oppose it for self-serving reasons.
If instead of imminent strikes the West and its allies in the Gulf offered Moscow mutually beneficial cooperation, the Kremlin would be much less likely to use its veto power in the UN Security Council.
But just as the US would do well to halt the inexorable drift towards war, Russia needs to give up the old view that geopolitics is a zero sum game with winners and losers. Ten years after Iraq, both need new geo-strategic thinking.
As long as each side privileges conflict and national self-interest over cooperation and respect for common rules, the ability of both to shape the global (dis)order will decline in favour of China and other emerging powers. Whether that will help the suffering Syrians and other oppressed people is unclear. But I am not optimistic.
Adrian Pabst is senior lecturer in politics at Britain's University of Kent and visiting professor at the Institut d'Etudes Politiques de Lille in France