President Donald Trump’s trade war is threatening US dairy exports just as the industry needs new markets for its booming production. America exported about $8.2 billion of dairy products last year, the second-highest on record, according to the US Department of Agriculture. Companies have built and expanded factories with the hope of shipping even more. China and Canada have already imposed retaliatory tariffs on some dairy from the US, raising the risk of crippling the industry’s overseas prospects.