IAEA inspection teams have reported that Iran so far is in compliance with the treaty terms.  And, if they are not, and are intent on building a weapon, they are starting from way back than where they were when the treaty was signed.  For example, the number of available centrifuges has been drastically reduced.  Those left would have to be certified to not be susceptible to the US-Israeli STUXNET malware, and good luck getting the controllers to do that.  The Iranian inventory of fissile U-235 has been severely reduced as well.  The heavy water reactor at Arak has been disabled.
If Trump and his cronies believe the Europeans plus Russia and China will reimpose sanctions, they are dreaming in technicolour (not for the 1st time).  Too many trade possibilities for the Euros, for one thing.  And it would demonstrate, not for the 1st time, that an American signature on a treaty is worthless.
Not to mention the regional nuclear arms race.