Even supposing Iran developed a nuclear weapon, their ability to engage in nuclear retaliation depends on (a) the number of warheads, (b) the available delivery mechanisms
An Iran which had only a handful of warheads, and rather limited delivery mechanisms (few or no ICBMs, no SLBMs, no long-range bomber capability) might find its ability to engage in nuclear retaliation against the US extremely limited
Even attempting to use nuclear weapons against Israel or regional US allies, there would be a massive attempt by Israel/US/allies to intercept any nuclear armed missile before it reached its destination
People argued missile defence (as in Reagan's "Star Wars") would never work against the Soviets because they could always just overwhelm it given the superabundance of warheads and delivery systems they had. The same logic does not apply to Iran, because even if it did build a nuke, initially it would only have a handful. Only if they were allowed to build out their nuclear arsenal and delivery systems without intervention, over an extended period, might that eventually come true.
