The Iran deal was an amazing bit of negotiation, the US paid Iran their own money in exchange for getting UN inspectors on the ground.
Trump undermined it as part of his campaigning and one of his first acts in office was try and get it overturned (which he couldn't legally as Iran was complying with the terms).
If he'd worked with them in good faith who knows what would have happened, as it is Iran has had ten months to work on whatever they've been working on knowing that the deal was doomed.
Of course it wasn't perfect, the UN inspectors were frustrated that they didn't have the access that they needed, but having people on the ground and having people at the table talking to you is much better than embarking in a flame war on twitter.
Yep, that's why I said they've had ten months of Trump telling them the deal is off to work on it. Much like how Obamacare is collapsing in some states through neglect we have no idea if Iran would or wouldn't have tested a ballistic missile if the US was dealing with them in good faith.
Although, Iran is apparently in compliance to the words of the agreement if not the spirit. The point is if he wasn't busy tweeting about how horrible they are and actually tried to make one of these deals that he claims he's so good at making he could do something about it. He can't do anything about North Korea because they have no diplomatic relationship, and he can't go to war with them because geographically they can cause devestation to US allies.