Donald Trump's politics have shifted considerably over his decades in the public sphere. But one thing he has been consistent on, since the 1980s, is his belief that tariffs are an effective means of boosting the US economy. Now, he's staking his presidency on his being right. At his Rose Garden event at the White House, surrounded by friends, conservative politicians and cabinet secretaries, Trump announced sweeping new tariffs on a broad range of countries – allies, competitors and adversaries alike. In a speech that was equal parts celebration and self-congratulation, regularly punctuated by applause from the crowd, the president...