In the previous post, Mutually-Assured Destruction, we looked at the payoff matrix for MAD, motivated by Iran's desire to acquire nuclear weapons.
However, the Iran case is quite asymmetric and the payoff matrix looks something like the above.
If the US does nothing, then Iran gets the bomb and scores +5, while the US has to deal with the negative foreign policy implications and scores -x (how big is x?).
The US therefore prefers Iran not to get the bomb, which motivates a first strike. If the US gets away with it, Iran scores -5 and the US scores +1 as its foreign policy problem has now gone away.
If however Iran retaliates: massive destabilisation in Iraq; attacks on Israel via Hezbollah in Lebanon; or more generically via a wider Islamic/world reaction against the US; then Iran would get battered again and scores -10 but the US has a negative outcome of -y.
Given a credible posture that Iran would retaliate, the US has to figure out whether -y is worse than -x, and Iran has to persuade the US that it would not take an attack lying down. The latter case is a lot easier to make than second-strike retaliation in the MAD case, although Iran would no doubt come off even worse if it retaliated.
For the US there are no stable good outcomes, and it's still a case of a "subgame perfect Nash equilibrium".
Arguably, there is the missing option where 'the West' persuades Iran not to acquire nuclear weapons, and 'play nice'. But that's a different game.