European leaders said Monday that they may have to stand alone without the United States once Donald Trump enters office, raising the prospect of an unprecedented breach in transatlantic relations after Trump’s comments that the European Union is bound for a breakup and that NATO is obsolete. Trump said in a weekend interview with the Times of London and Germany’s Bild newspaper that the 28-nation European Union was a vehicle for German interests and said that he was indifferent to the bloc’s fate.