US President Donald Trump has left the 2015 deal that limited Iran’s ability to build nuclear weapons. Although Iran has complied with every requirement of the Joint Comprehensive Plan of Action (JCPOA), the US won’t keep its side of the agreement: Trump plans to re-impose heavy trade sanctions.

The other parties to the deal – the UK, France, Germany, China and Russia – still support it. But the threat of US penalties is expected to massively discourage firms from trade and investment in Iran, which could cause the JCPOA …