As I said before, Israel already had nuclear weapon years before even some major powers have it. How do you stop Israel to have nuclear weapon at that time?
It also did not cause a chain reaction. By contrast, Saudi Arabia leadership has stated explicitly and repeatedly that if Iran gets a nuclear weapon, Saudi Arabia will also build one.
Iran problem is in front of us right now. It can still be prevented from developing nuclear weapon. I also already provided some suggestion so Iran can have nuclear guarantee in my first post in this thread.
Deterrence between two nations (like the US and Soviet Union, or India and Pakistan) is already difficult. Deterrence among four hostile nations (Israel, Iran, Saudi Arabia, Turkey) is probably more difficult to stabilize. If a missile is launched in a four-way nuclear countries in Middle East, how do you verify who fired it before your own country is wiped out?
We do not prevent Iran from getting a nuclear weapon because we believe the current status quo is fair. We prevent it because a highly proliferated, multi-polar nuclear Middle East will probably dramatically increases the statistical probability of a nuclear detonation. In the probability of preventing nuclear winter and kill millions of people, keeping the number of country with nuclear weapons to an absolute minimum is probably better, regardless of how unfair the baseline is.