World News

I mean if you're assuming the US does land-based launches from the continental US perhaps.

However most likely they'd use air deployed or sub launched weapons to avoid just that and to give Iran less time to react. Most likely launch locations for sub based weapons would be somewhere in the mid-south Atlantic, the med, or west pacific off Africa. The Russians are not capable of doing anything but satellite sensing about that, it wouldn't enter their early warning radars. They'd probably notice it, bring their forces to full alert (they're already nearly at it) but not presume anything and once they started landing in Iran it would be clear as if Isn'treal nuking them beforehand wouldn't make it clear.

Most likely the US would prefer air launched because they could then fly a plane from southern Europe loaded up with some on missiles, drop in the straits waters and turn away. Again Russia and China wouldn't see these things happening. They don't have some magical beyond the horizon radar systems.

The only things that they can react to are early warning radar systems which require the projectiles to enter within a certain distance of their airspace (and by the way you can calculate trajectory and something aimed at Iran, assuming it's fired from a respectful location (e.g. not the north atlantic where it could easily turn and hit Moscow) AND of course satellite imaging. Satellite imaging has trouble catching sub launches unless you already know where the subs are and are actively tracking them or are doing wide area sweeps.

It would freak both of them out and as I said they'd go to full alert, finger two inches from the button type situation but assuming the US isn't absolutely ridiculously incompetent and firing these things anywhere near those two there won't be an immediate reaction of "let's launch everything". The US would likely call Russia and perhaps China a minute before launch and tell them they're not firing at either of them but at Iran and that would further ease things. Russia would be more alarmed given their proximity and would have their missiles ready and would certainly be forceful in the aftermath of not wanting US/NATO occupation of Iran but again wouldn't stick their neck out for them most likely. Once the missiles are in the air nothing Russia can do, they can avenge them by nuking NATO bases but that won't undo Iran being wiped from the map or do anything but tempt the US to strike back.