That would bring Iran one country closer to Israel. I can’t see the US pouring in troops to defend Iraq in the face of an Iranian Invasion.
Iran has plenty of trouble inside its own borders, and it’s all the time. Iran is also not just Iranian — like all its neighbors it’s cut into ethnic groups. And everyone is more and more heavily armed. The upshot is, they probably couldn’t survive an invasion, even if they had the money to buy what they need (and they don’t, the oil price has dropped 20 percent during 2014) and could obtain the materiel despite sanctions (they probably can’t).