Will the US back down or attack Iran risking war with Russia ?

There will be no attack,as was projected for the better part of 2006. Putin has made his views known - publicly - before this,and he has certainly communicated them to the U.S. government via diplomatic channels. No,it would not be worth a war. Iran has done nothing to afflict either the U.S. nor it's little spoiled brat of a sponsored-state.