In many ways the SCO is set up similarly to NATO and they have a mutual defence pact. It may not be as binding as NATO’s article 5 but it does seem that these nations have begun promising much closer military defence in the event of attacks from outside agents. Therefore any attack on Iran could see its SCO allies forced to come to its defence. In the months after 9/11 the US were able to set up bases in Uzbekistan from which it launched its invasion of Afghanistan. But at the urging of the SCO the Uzbeks have managed to close all the US bases and are now getting much closer to China through this organisation. So now the region is once more China and Russia’s domain.
Should the US be crazy enough to attack Iran it may see the Central Asian republics being sent to their defence. China appears to be getting ready for this potential and is drilling for this occurrence. The SCO holds regular joint military drills each year but this year has held numerous large scale war games with China and its Central Asian allies since the Lebanon crisis of July 06. Again this appears to be a military build up occurring in a potential theatre of this global war.
If such a clash were to occur then the very real risk would be that the US would then demand that NATO be used as a counter to this. NATO troops are already in neighbouring Afghanistan so the risk of the SCO coming into direct conflict with NATO rises. We could then see the broader world war shaping up as a giant clash between SCO and NATO nations.