what else could defeat Iran’s mountain fortifications?
Nukes can't do that, actually.
They would have to deplete their very difficult to replace nuke stockpile by nuking every square-kilometer of the Iranian countryside to be sure there isn't enough IRGC units left to fight back, especially if they use smaller tactical nukes. Any less than that, and all that would do is make the IRGC take the gloves off completely and dunk on the US and allies even harder than they already do. And not even the US has anywhere enough nukes for that. Nukes are good at destroying a large amount of targets concentrated in a small area[^1], but they are terrible at destroying a large number of targets scattered over a large area. Which the IRGC is.
[^1]: Actually, this disputable is some cases. Especially if you want to be efficient about it. Nukes are not efficient. There's also all the radioactivity issues you'd have to deal with afterward.
Nukes can't do that, actually.
They would have to deplete their very difficult to replace nuke stockpile by nuking every square-kilometer of the Iranian countryside to be sure there isn't enough IRGC units left to fight back, especially if they use smaller tactical nukes. Any less than that, and all that would do is make the IRGC take the gloves off completely and dunk on the US and allies even harder than they already do. And not even the US has anywhere enough nukes for that. Nukes are good at destroying a large amount of targets concentrated in a small area[^1], but they are terrible at destroying a large number of targets scattered over a large area. Which the IRGC is.
[^1]: Actually, this disputable is some cases. Especially if you want to be efficient about it. Nukes are not efficient. There's also all the radioactivity issues you'd have to deal with afterward.