Its a crazy world we live in, where a country with literally 50x less financial resources and a fraction of the military budget or capability can provoke the larger power, knowing that the larger power can't really do anything back, or at least any action taken back would hurt the larger power far more.
Its a very unusual position historically to be in, but its where we are. Obviously there will be some american's who will think 'well, in all out war, we will crush them!' and they would be right, but all out war is an impossibility, it doesn't exist like it used to, and likely never will.
American can't use their nuclear dominance for obvious international reasons..
They can't use their dominance in numbers/population, because the value of human life has, for the first time, vastly exceeding the value of war/conquest in the past century (in western nations)
They can't use their military and technological superiority, because you can never truly win against an opponent who would die before giving up, and eventually it will become too costly in terms of life/money to keep going (see basically every US war since WW2)
All, the absolute most they could do is send in a few strikes, bomb a few targets.. but for what? A show of force? Its not going to threaten a nation like Iran away from actions, it will provoke them to do more.. so why bother?
There is no positive path forward for the US here, and they know it. Iraq/Afghanistan were the last straws.. they showed that even against incredibly week opponents, you can't win if they are prepared to die, and they showed that the US population really doesn't have the resolve to handle prolonged conflicts. There is no desire for war with Iran among the American people, and any president who started one would become the most unpopular president of the past century, given the costs that it would incur.
Its a crazy time we live in.. in the previous era, Iran would be finished by now, it would be defeated and occupied (as it was), but in this era.. Iran actually has the advantage over the larger nations in conflict situations like these.. and it seems to fully know it, and is prepared to act on it.