When Donald Trump warned Iran on April 7 that “a whole civilisation will die tonight,” a European diplomat in Washington said his government wanted an urgent answer to a chilling question: Was the US president contemplating the use of a nuclear weapon? Across Europe and Asia, the concern went beyond whether Trump’s apocalyptic threat was real or bluster. One fear, the diplomat said, was that Russia could seize the moment to justify similar threats in Ukraine, triggering a nuclear crisis on two continents. European governments immediately sought reassurance through a traditional channel: the US State Department. But according to the diplomat, officials there gave an unsettling response: they didn’t know what Trump meant or what actions his words might portend. The previously unreported episode points to a historic breakdown in American diplomacy. At a moment when a uniquely unpredictable US president is rattling markets and capitals with dramatic pronouncements, governments around the world are scrambling for ...