US won't be entering a full blown war against Iran anytime soon, that would fuck over the ME big time, Iran is not a small time player, it is a regional power house who effectively controls Iraq, Syria and Lebanon with influence reaching Yemen and Bahrain.
Their poorest and least equipped militias in Yemen managed to stave off a billion dollar Saudi Machine and managed to strike their main airfields with home made SRBM's.
Their most elite equipped militias, Hezbollah, humiliated the IDF back in 2006 and changed the course in their favor in Syria.
I have no doubt that in the case of a full out war, Khamenei can call for all able bodied men across Shiadom for a holy defense much like Khomeini did and you'll have tens of thousands answering even more so than than they did back during the Iran-Iraq war. Some people like to paint the regime in Iran as unpopular, from first hand experience, I can certify that millions in Iran will view a US/Saudi attack as another Karbala.
The US can bomb via airstrikes and Iran can retaliate against he US fleet, problem is how willing are both sides to escalate.
As for sanctions, Iran survived worse during the Iran-Iraq war, Rouhani needs to keep the Europeans as allies but at the same time needs to send a message than Iran won't be fooled, we'll see tomorrow how it plays out.