I wonder if this war will lead to nuclear strikes. If Israel/America cannot sufficiently degrade Iran's AD, they are practically the only available solution to both the need to bomb large amounts of Iranian surface infrastructure and also to reach deep underground strongholds. It is too early at the moment, but after at least a month or two there may be talks about this option.
It may sound needlessly escalatory, but I want to remind all that Trump's two presidencies have been defined by just how many otherwise "sacred" norms he has broken and "infamous firsts" he has caused, not to mention Israel as the world's only openly genocidal state.
I suspect Israel will want to keep its own nuclear weapons as an actual last resort for if their core territories are threatened by land invasion, but they may urge Trump to use American nukes to hasten the end of a war that would otherwise be his presidency's defining quagmire and failure.
My primary concern at the moment is that the global response would be weak. The events of the last decade have shown that, despite all the rhetoric to the contrary, America can in fact do whatever it wants and only the other superpowers can resist. The rest just grumble and acquiesce. From unilateral sanctions (countless) and blanket trade wars (two) to bullying allies (Greenland) to assassinations (Soleimani) and kidnappings (Maduro), the world merely expresses the same shock and insults to Trump then go about their lives, offering only public speeches of hesitation to work with the Americans over the long-term. The only true response I can see is Russia using nukes in kind in Ukraine (should the tactical situation require it), but even that seems far-fetched with Putin's sentiment to the affected peoples.
Similarly, I do not believe the first nuclear strikes will hit population centres. They will try the underground nuclear facilities first. Only after they've gauged the relations cost would they decide to proceed with targeting cities. Iran has no real means to respond to nuclear escalation, and I do not see Russia nor China extending them a nuclear umbrella as Iran's fate does not affect their core interests. Similarly, I do not see how the Rest Of World could make Israel/America pay for breaking the nuclear taboo. The power imbalance is too lopsided for what ultimately amounts to just a moral line in the sand (as horrible as that sounds).