US President Donald Trump’s withdrawal from the 2015 Iran nuclear deal has undermined the position of key European allies. The move is unlikely to achieve its stated ambitions, but has stoked uncertainty for buyers of Iranian oil.

Mr Trump waxed lyrical about Iran’s pernicious regional activity on 8 May as he announced his decision to exit the Joint Comprehensive Plan of Action (JCPOA). He highlighted Iran’s support for Hezbollah and Hamas and said it “continues to...