A war with Iran will prove to be a nightmare that will haunt the world for a long time.
That part of the world is too complicated for the west to understand. Just like Iraq, it's not as cut and dry as the media would like to portray. You are not gonna overthrow the leader there and expect everything to be all right (which I doubt that was the aim of the USA and friends, they want discord there). It's a very, very, complicated place with a myriad of different views, opinions and agendas. It's gonna be a bloodbath if what happened in Iraq is repeated in Iran (and on a much bigger scale I might add). A much stronger terrorist group will rise from there, a force that would make an ISIS soldier blush, Russia would overrun the region with weapons (with the US, UK, and France..etc getting a piece of the pie). Europe will be filled with refugees and you can count on terrorists exploiting the chance to take a trip to the west. The hostility there is palpable, the two friends of the USA there (KSA and Israel) want nothing but to see Iran restrained.
Russia and China are watching from the shadows waiting for a war to start there. The USA and friends have been for a long time controlling the scene and directing the play. The beatstick that they have been swinging around have bloodied many heads and it won't come as a surprise if the ones with bloodied heads are holding a grudge. The way Russia is treating the countries who have less than a good impression of America and Friends is very savvy.