President Donald Trump will soon announce his decision on the Iran nuclear deal. He calls it the"worst deal ever" but on Tuesday night Theresa May told him it was "vital to regional security". Clearly the special relationship is under strain.Trump wants out of the deal because he believes it typifies Obama and his weak foreign policy. This is politically powerful, but it is diplomatically dangerous. With no alternative plan to contain Iran, he is risking an international crisis of his own creation.

Let's rewind a little to understand how dangerous the situation was before the Iran deal was signed. Iran was racing to nuclear breakout, and perceived as a clear and present danger by Middle East states that Iranian leaders marked out for destruction. Israel, which the Ayatollahs have promised to wipe out since 1979, threatened to go it alone with a military option if international action was not forthcoming. This was no idle threat: Israel destroyed an Iraqi nuclear facility in 1981 and in 2007 attacked a Syrian chemical weapons factory.

But then, in 2015, Iran signed a landmark agreement with Iran, the US, France, the UK, Russia, China, and Germany. Iran agreed to suspend its nuclear programme in return for an end to crippling financial sanctions. The US president would have to certify to Congress every 90 days that Iran is in compliance. Now Trump is considering whether he should instead announce that Iran is violating the agreement.