US sanctioning power is kind of interesting because US officials didn't really know they had it and kind of stumbled into it after 9/11, which made them realize that because the dollar is the international currency and they ultimately control settlement in dollars, even foreign companies would cooperate without asking questions and even go beyond what US officials want them to do because the costs of being cut off from the dollar system are so enormous that overcompliance is actually rational from their point of view.
But the same facts that give US officials so much power also imply they can't actually control that power, because it means that while they can sometimes lift the sanctions themselves, it's much harder to undo their consequences. The problem is that, as a result, sanctions lose part of their effectiveness as a foreign policy tool, because it reduces the incentives of target countries to change their behavior since US officials can't credibly promise to undo the consequences of sanctions. I think this actually explains a lot about Iran's current behavior.