Is the Iran nuclear deal effectively dead? Three questions.

The Iran nuclear agreement, formally known as the Joint Comprehensive Plan of Action (JCPOA), took effect in January 2016. Negotiated between Iran and the five permanent members of the U.N. Security Council – China, France, Russia, the United Kingdom, and the United States – plus Germany, the deal imposed restrictions on Iran’s civilian nuclear enrichment program in return for relief from nuclear-related economic sanctions.

Now the agreement appears to be falling apart. President Donald Trump withdrew the U.S. from the JCPOA in May 2018, saying that among other things

What was the Iran deal supposed to do? Why is the deal in trouble?What happens now?