The pro Israeli optimist in me wants to believe that if Israel attacked Iran, it by miracle will turn into `cakewalk`. But realist in me totally agrees with you, an you have not even mentioned possible Iranian proxies involvement. However, even understanding & considering all the awful downsides that exist, if Iran continues to develop nuclear WEAPONS along delivery mechanisms, there will be point when Israel will have no other choice whatsoever to attack, alone or not. There is only one worse case scenario exists but that is enough, & preemptive attack is not an aggression but the psychological (& very much probably physical) survival without experiencing first hand strike. I also believe that even if Israel undertook any actions alone, it would not take long for others to `join` in some capacity, particularly US.
from the article: Think tank: Israel could attack Iran's nuclear program alone