arms go to Iran already in progress as Russia

and China are already selling weapons to Iran and will continue in the foreseeable future. For China to retaliate against US would be to unleash North Korea against Japan, take over Taiwan, and other strategical options. Russia would only be logistical help to China and probably glad to see that US and China fight each other limitedly (no nukes) yet. Europe would probably stand by and be neutral so it can supply whatever side (US or China). Then it would depend who will escalate battle in to full war and nuke war. Both US and China cannot afford a war like this therefore hard to imagine fight a full war or nuke war.

War against Iran remotely possible but US need support politically around the world. If it doesn't get at the Europe Union into it, it would be a more disaster than post war Iraq are. You think the American people is ready for another war in the mideast after going through with Iraq? Domestic politics are also getting hot within US government and many issues about Iraq and Iraq abuses are still not resolved. Despite the media propaganda, more and more US citizens are getting restless everyday. The slogan "Support the Troops" are wearing thin and slogans such as "Change the government" are getting stronger.