At best, an "agreement" will be made that is no better than the one that was torn up. This will be used as propaganda to suggest it was all worthwhile, and victory has been achieved. That is despite nothing having been changed. That is, at best.
At worse, as suggested in this thread, nuclear weapons will deployed. Millions will die. But not simply millions in Iran - the winds don't honor borders. In fact, people will suffer for decades, long after the reasons for this war have been forgotten. It will normalize the use of nuclear weapons - I mean, if they can be used in Iran, why not Ukraine? And so, we spiral to mutual destruction. US allies will shun the nation. More nations will seek the capability.
How can that be right, or just? If hating another country was reason enough for nuclear Armageddon, then we'd not be writing back and forth. We'd be living in caves.