OTTAWA — The security fences are coming down. And the world leaders have jetted off. But for Prime Minister Justin Trudeau of Canada, the troubles have just begun. After the disastrous finale of the Group of 7 summit meeting, Mr. Trudeau is now caught in a tight spot between the unpredictable President Trump and the powerful Canadian dairy industry, the current target of Mr. Trump’s escalating trade threats. The prime minister’s challenge is how to manage both the most important Canadian ally and his own domestic politics. …And despite Mr. Trump’s unpopularity in Canada, Mr. Trudeau scrupulously avoided criticizing the president. Mr. Trump rewarded the effort with his hard-line positions on reworking the North American Free Trade Agreement, duties on Canadian lumber, steel and aluminum, and insistence that national security concerns justified those measures — to Canadians, an insulting position to take.