With a stroke of his pen US President Donald Trump has jeopardised the one agreement – good or bad – that seeks to constrain Iran’s nuclear ambitions.He launched a scathing assault on the deal and its deficiencies. But he offered no alternative policy to put in its place. He has put US diplomacy on a collision course with some of Washington’s closest allies. And some fear that he may have brought a new and catastrophic regional war in the Middle East that much closer. The 2015 Iran nuclear deal – or JCPOA (Joint Comprehensive Plan of Action) to give it its technical name – is not quite dead. But it is most certainly on life support. Much now depends upon how the Iranians respond. President Hassan Rouhani has been a staunch supporter of the agreement. He appears to want to try to explore with the Europeans and others the possibilities of keeping it alive. But he faces strong internal opposition from hardliners, some of whom want Iran to break out not just from the JCPOA – but from the Nuclear Non-Proliferation Treaty (NPT) altogether. President Trump appears to have cut the ground from under his Iranian counterpart’s feet.