Comment by krisoft

Unilateraly on the level of countries. The other signatories (China, France, Russia, the U.K., Germany and the EU) believed that the deal was good and Iran was holding up their end of the bargain at that time.

If the USA government had credible evidence that it is not so, they could have picked up the phone and presented their case to the other signatories. Or at least to their allies. Then once those countries were convinced that something is off they could have withdrawn together from the agreement. Would have less of a terrible optics than how it went down.