<<They need a deterant to prevent us from do ing what ?>>
It's a good question. I'm not sure. Maybe it's just to stiffen their own backs, like sometimes in the middle of the night you hear a noise in the kitchen and you get up to investigate and here (in non-gun-owning Canada) you might think, Jeeze, wish I had a gun in my hand right now. They might just feel more confident standing up to the U.S. in their dealings with them if they had a nuclear arsenal.
The conventional answer is often used. I'm not sure if it's correct or not. But the conventional answer is: to deter the U.S. from invasion or some lesser form of aggression. The argument is buttressed by the fact that North Korea, which had nukes, was not invaded or attacked by the U.S., while Iraq and Afghanistan, which did not, were both invaded and occupied.
<<They are not likely to grow a deterant better than the USSR had and we struggled with the USSR for fifty years waveing our Atomic deterants at each other.>>
Well, in the eyes of the U.S.S.R., the deterrent worked because the U.S. was not able to take advantage of its nuclear monopoly to launch a first-strike nuclear attack. When the monopoly was broken, the U.S. was not able to use nuclear blackmail against the Soviets.
<<We still have thousands of warheads left over from that , so do the Russians .
<<What does our deterant deter them from?>>
I think you're confusing who's got the deterrent and who hasn't. The U.S. bomb was not developed as a deterrent to anybody. It was developed and used as a weapon of war. It was the Russian bomb that was the deterrent.
<<Aren't they just as sure that they will never use these wepons as we are?>>
Who is "they?" Russians or Iranians?
I think it's undeniable that the Iranian bomb will be a comfort factor, they'll feel better and more secure with it than without it, just like me in the kitchen at 3:00 AM investigating a suspicious noise, I'd feel more secure with a gun in my hand. Rational? No. Effective? Yes.
The contrasting examples of North Korea, Iraq and Afghanistan are also very instructive. You can ask all the questions you like, would the U.S. do A if Iran did B? and so on, but at the end of the day, nobody can answer the hypotheticals with any real assurance, but one thing everybody DOES know and that is that North Korea, with nukes, told the U.S.A. to go fuck itself, and nothing happened, whereas Iraq and Afghanistan, without nukes, both got invaded and occupied. That in itself would be good reason for Iran to get nukes, the bigger the better, and as fast as they can, deterrence or no deterrence.