Putin would definitely step in to defend Iran. I think covert regime change, like they did in 1954, would work best. They might use the confusion of a fog of war during these targeted attacks to install a new government somehow. I really don't think we'll see an Iraq-style invasion of Iran, unless Israel gets nuked or something super dramatic like that. But Iran knows that'd be suicide, so they wouldn't do it. That's how it looks to me, anyway.