President Trump threatened on Tuesday to unleash “fire and fury” against North Korea if it endangered the United States, as tensions with the isolated and impoverished nuclear-armed state escalated into perhaps the most serious foreign policy challenge yet of his administration. In chilling language that evoked the horror of a nuclear exchange, Mr. Trump sought to deter North Korea from any actions that would put Americans at risk. But it was not clear what specifically would cross his line. Administration officials have said that a pre-emptive military strike, while a last resort, is among the options they have made available to the president.
