The US is just one nation -- what effect will US withdrawal have on Iran, assuming that other countries stick to the deal and impose no sanctions?
There is concern that as the relationship between the US and Iran cools further, Iran might cleave closer to Russia and China. `source
Mohsen Milani's "The Making of Iran’s Islamic Revolution" is a good read, and I think it does a good job of telling the 20th century story (particularly how external powers manipulated the country). It's a good read now to understand how similar moves have played out in the past.