Reports indicate that Trump plans to “de-certify” the Iran deal, which would set in motion steps to reimpose American sanctions on Iran. Regardless of what you think of the Iran regime, isolated sanctions by the U.S. alone, without international cooperation, will be ineffectual. As I wrote in 2015 (and re-posted this February), this is actually the reason why the Iran deal made sense from the beginning: Russia, China, and Germany were preparing to end their participation in sanctions once Iran demonstrated that it was not pursuing a military nuclear program. At that point, the Obama administration had two choices: 1) symbolically support its own sanctions while the rest of the world ended their sanctions and allowed a weak inspection program (result: we make ourselves feel good, but no real world effect on Iran’s nuclear capability); or 2) make a deal to end sanctions for a stronger inspection program (result: taking a political hit from the right, but making a real world impact on Iran’s nuclear capability).
Obama chose #2 wisely. Trump is choosing #1 to score empty political points, in a counterproductive move that could make Iran more dangerous, and would make Iran’s moderates weaker.