Speaking to reporters on his Middle East diplomatic tour, Donald Trump hinted at what could be his biggest foreign policy achievement to date. A nuclear deal with Iran is ‘close’, he said. Tehran has ‘sort of’ agreed to curbing its suspected clandestine atomic weapons programme.

The US and Iran have now had four rounds of indirect negotiations in Oman, and although the content has remained confidential, the atmosphere between the two sides has been candid but amicable, raising expectations that a deal to stop Tehran ‘breaking out’ and building a nuclear bomb could be brokered diplomatically without the need for Trump...