> Why would action [by Israel against Iran] be impossible without US
> involvement?
1. Israel would have to send bombers across Iraqi airspace, which is controlled by the US.
2. A single bombing run would be insufficient: the Iranians have spread out their nuclear development sites, so a number of bombing runs would be necessary. So unless those could be done very quickly, Iran would have time to respond in ways that would make other countries feel the pain (e.g. impede oil tanker traffic) in an attempt to bring indirect pressure on Israel to stop half-way.
3. In general, Iran could respond in ways that piss off large parts of the world. Israel would need the US to back it up.