If a guy tells you there is a 5% chance he might kill you someday in the future, do you have a moral justification to kill him today?
My answer is yes. You would have a big legal problem, but from a moral standpoint, it's close enough to self defense in my book.
But what if the guy is a bit hard to read on the subject of whether he might kill you in the future. Let's say you can't estimate the odds of it happening because you're not even sure if he means it, or maybe he has some motive in making you think he might. There's no way to know what's in his head. He's broken no laws because he speaks indirectly as in "Someone will kill you soon" as opposed to "I plan to kill you soon." Can you kill that guy and be morally (but not legally) justified?
I say yes again. You might be killing a relatively innocent man, but accidents aren't immoral.
Now let's say you know that if you kill this one guy, his brothers and sisters will hunt you down and get revenge. It's that sort of family. So killing this guy will only make things worse. But suppose you had a good opportunity to kill all of the brothers and sisters, by bombing their family reunion. You'd get all of the dangerous ones plus a bunch of total innocents. Would that be morally (but not legally) justified as self defense?
I say yes. The guy who threatened you is the one who put his family at risk. And if you misread the threat, that's a tragedy, but not immoral. Accidents are accidents, no matter how horrible.
All analogies are flawed, but that's how I see Israel looking at Iran. From my safe little chair in California, it seems highly unlikely that Iran would unleash a nuke, directly or through proxies, at Israel. But is there a 5% chance it might happen someday? Maybe. There's no way to estimate that sort of thing.
If I lived in Israel, I would feel morally (but not legally) justified in attacking Iran to reduce a hypothetical 5% risk of nuclear annihilation. But that's just the moral argument. On a practical level, I have a hard time imagining a massive attack on Iran making Israel safer in the long run. Still, if I were within Iranian missile range, a 5% risk would look exactly like a 90% risk to me. I'd treat those risks as if they were the same.
Here's an interesting view on Iran from Newsweek's Fareed Zakaria. Does his argument look like something you once read elsewhere?