At this topic I like to discuss How Americans would attack Iran and how Iran would answer to that attack?
from which side
I see 2 possible fronts with (in)direct access to Iran
-First front would be Afghanistan as the USA has a significant presence there.
-second front could be Kuwait, as only a small part of Iraq would have to be crossed in order to reach Iran, to support this front they would probalby use their strong navy and air force, stationed in Saudi Arabia, UAE and the like. Maybe Iraq itself can be used, as we can doubt the capability of Iraq to defend itself, but the USA would face a lot of opposition in the form of guerilla warfare.
Perhaps the Kurds in Syria would be more friendly and host the USA a little, but then you can extend the conflict to Syria as well, as I don't see Assad just sitting there.
I don't see the USA using Turkmenistan or the other 'stans' around it, as Russia has influence there.
Turkey will not allow the USA to use their territory, unless they have a serious beef with Iran on their own, which they do not. Same can be applied to Pakistan and Pakistan has India to worry about.
I think the Caucasus cannot be used for obvious reasons.
Everything they can possibly use, obvious weapons would be cruise missiles, jet fighters, (stealth)bombers, Aircraft carriers and submarines. Perhaps the USA could even use conventional warheads on their ballistic missilies.
The only real ally would be Hezbollah and other foreign (Shia) muslims fighting for Iran. Also a few volunteers from all over the world can be expected. Perhaps mercenaries can be recruited.
Russia would definitely use Iran as proxy against the Usa, by diplomatic support and delivery of weapons and supplies. Perhaps they could even send a few hunderd 'advisers' or 'volunteers', I think Russian support would be reasonbly significant.
Then China would also deliver aid in the form of weapons and give diplomatic support, but probably in a lesser intensity than Russia.
USA would most likely have the Saudis, UAE and NATO nations aiding them
The war ending in an American victory is quite unlikely, if not a pyrrhic victory, I think. Iran has the Caspian sea to use as a supply like from Russia and especially the west of Iran is very difficult to traverse.