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.

“Trudeau is in a very difficult place because the G-7 summit signals the approach the Trudeau government took to deal with the Trump administration has failed,” said Wesley Wark, a professor of public and international affairs at the University of Ottawa. “The charm offensive has produced little of what the prime minister hoped.”

Even before Mr. Trump came to power, Mr. Trudeau, and his closest aides and cabinet members rushed to connect with the new president and his advisers. And despite Mr. Trump’s unpopularity in Canada, Mr. Trudeau scrupulously avoided criticizing the president.