US President Donald Trump has called the landmark nuclear agreement in which Iran agreed to limit its nuclear programme in exchange for limited sanctions relief the “worst deal ever” and threatened to scrap it. Mr Trump is expected to refuse to certify the deal to Congress by an October 15 deadline and initiate stronger moves against Iran. That will not put the US in automatic violation of the deal, but what happens next is likely to be complicated, messy and potentially explosive. Who supports the deal? The 2015 deal, known as the Joint Comprehensive Plan of Action, was the crowning foreign policy achievement of the Obama administration. The four other permanent members of the UN Security Council — China, France, Russia and the UK — joined Germany and the EU in signing on to the deal with Iran. The process was endorsed into international law by the UN Security Council. Today, all parties agree with assessments from the International Atomic Energy Agency, a UN body, that Iran is in compliance with the deal.  What is the October 15 deadline about? Congress, which was never asked to ratify the deal because of its near-uniform antipathy for it, sought a role nonetheless: it passed the 2015 Iran Nuclear Agreement Review Act, which requires the sitting administration to certify US commitment to the deal every 90 days, including whether Iran remains compliant and whether the deal remains vital to US national security interests. The next deadline falls on October 15. What happens if Mr Trump does not certify the deal by October 15? If Mr Trump does not, as expected, certify the deal to Congress by that date, Congress would then have 60 days to initiate legislation under expedited procedures that could “snap back” the nuclear sanctions it continually waives on Iran. This would put the US in violation of the JCPOA, which could fall apart as a result.