The general understanding in the intelligence community is that Israel doesn't attack Iran without at least the tacit approval of the U.S. Iran will declare it a U.S. attack regardless. It's a risky gambit, but not all that risky if you look at Iran's history of angry threats followed by doing nothing. Still, these times are a bit more fraught.
The problem is that Iran has a large population of young, educated professionals who have generally been pro-Western in recent years, but have not been able to overthrow the mullahs who rule as a repressive theocracy with a huge Persian chip on their shoulder. You do not want to "level the place" because that's the quickest way to get potential allies to be your sworn enemy for generations.
It's kinda funny how our rightwing xenophobes are always rooting for the overthrow of Islamic rulers and don't notice that they are siding with the college educated, science-friendly, culturally liberal, global-minded, and demonstration-happy folks they despise when they're Americans.