I see your point. The problem is Iran already military helping Assad's government: Iran confirms it has forces in Syria and will take military action if pushed | World news | The Guardian
"Iran has confirmed for the first time that forces from its revolutionary guards corps (IRGC) are in Syria helping Bashar al-Assad's government crush rebels, and warned that it would get involved militarily if its Arab ally came under attack."
Well, there are lots of precedents to that. The US came to be involved with a Civil War in Afghanistan! What had the US to do with who among the various tribes would control the other tribes?! Afghanistan is a very poor country. There's no oil in Afghanistan. Just for fun? Maybe, to catch Bin Laden? Have the US caught Bin Laden in Afghanistan, after all?
I'm not 100% sure what China has to do with Iran (the slightly cheaper oil or reliable oil supply, maybe?), but they have clearly stated their military support. If you know Chinese, they are not just talkative for fun..