The US and the Soviet Union were in a nuclear stalemate for decades. It was never necessary for either side to wage a first strike nuclear attack against the other. It came close to happening a few times, as much because of the US as because of the Soviet Union. But the US government had us believe that the Soviets were evil madmen, who didn't need any reason to attack us other than the same sort of thinking that you are attributing to the Iranian Mullahs.

The reason countries like Iran want to arm themselves with nuclar weapons is because they know from the behavior of the US, that they are relatively safe from US meddling if they have nuclear weapons. I don't necessarily think that it's a good idea for the Mullahs to have nuclear weapons. However, if Israel chose to wage a first strike nuclear attack on Iran, I would pretty much guarantee you that it would be for reasons other than the idea that they expected a first strike nuclear attack against them coming from the Mullahs of Iran.