On paper, the US constitution is a thing of beauty. But the would-be emperor in Washington has revealed its great weaknessAmerica’s big birthday has come at a bad time. On Saturday it will be a divided nation that marks 250 years since 13 North American colonies declared their independence from the Great Britain of George III. Many will be anxious that the republic they established that day is fragile – not least because of the would-be emperor in the White House.Some will console themselves that hope and angst have always been intertwined in the American story. From the very start,...