A degree of realism is necessary when judging how much influence Europe can bring to bear on Mr Trump. The US president’s claim on Monday that the war is reaching its endgame was designed to calm markets rather than mollify supposed allies. But while doing what is necessary to protect their interests in the Gulf, European governments also have a duty to stand up robustly for multilateral principles and norms which – however imperfectly adhered to – used to make the world a safer place.