Iran: what can Trump do, and with what consequences?

Donald Trump has threatened to pull out of the nuclear deal with Iran.The US president already refused last October to "certify" the action plan signed by Iran and the five major world powers -- the United States, Russia, China, France and the United Kingdom, even though international inspectors confirmed the Islamic Republic was meeting its obligations.In return for steps taken by Iran, Washington suspended sanctions imposed on the country over its nuclear program.