They nuke them first. The US has been itching to use them for decades and will finally have the perfect excuse. You can't really build a nuke while under active attack by a nuclear power (two nuclear powers, one a vassal of the other) who need to destroy you for their plans of maintaining global hegemony. At that point they have nothing to lose from hitting you with a nuke to stop you using yours. You need several nukes BEFORE they launch the war and a way to deliver them to ward them off.
Once the war has started a threat likely won't scare them off and really your only choice is revenge, is firing it at them and killing a bunch of their people because they're already going to be nuking you so you get to die happy you took a million of them with you or whatever.
One thing I'll note is that the US and the zionist entity are likely to target and attempt to take out Iranian missile production and launch sites as well as mobile launchers as the next phase. The zionists are of course annoyed by all the missiles landing in the entity and more than that it is stretching resources thin for the empire firing off all those missiles.
Cynic in me says the US is letting Iran hit the zionists with their advanced missiles in order to study them in order to perfect anti-missile systems for their own future use. They simply need that data and are happy to see some settlers die for it. But once they have that data and once Iran starts hitting them in earnest assuming they have a stockpile and real capability to actively produce enough to meet their needs (a big if, I am a little doubtful but we'll see) they'll move to take out those capabilities meaning it will be harder even if Iran gets a nuke to successfully launch and deliver it. Nuking the entity would drive zionists up a wall with rage but after they nuke Iran back and destroy it totally the settlers will return to the zionist entity and I'm afraid it likely will survive.
This is the problem with not having control of your airspace at all. You can do as Yemen does and produce a small amount and harass them with them but not enough to win a war or inflict enough pain to collapse the entity or drive the US to sue for peace. A destabilized middle east/west Asia suits the US alright as we produce plenty of oil and have Canada as well, it'll fuck Europe and China HARD and drive up costs for your average person but given how much consumption is already driven by the top 10% they may think it really doesn't matter to siphon more wealth to the top and impoverish the proles even harder.
Because they would prefer to avoid a direct war. They'd prefer a Syria/Libya type situation of engineered moderate rebels and selective strikes to maximize chaos and breakdown. They already have Azerbaijan totally in the zionist/NATO pockets hosting their planes and harassing Russia just across the border which they can use to aid with arming these. They hope to inspire ethnic tensions to create an actual fall of Iran that's durable and removes it as any kind of a power player for decades.