As President Trump rolled out his latest round of tariffs on Thursday, he fell again into what has become a familiar, if surprising, pattern — favoring Mexico and stiffing Canada.

Even as he announced sweeping tariffs for much of the world, Mr. Trump offered Mexico a 90-day reprieve, pending further negotiations. Then for Canada, America’s largest export market, he raised general tariffs to 35 percent from 25 percent.

Even worse for Canada, its new rate went into effect shortly after midnight, while new tariffs against other nations will take effect in a week.

The reasons for the imbalance in the president’s treatment of America’s two closest trading partners was not immediately apparent. But many Canadians believe that it is part of Mr. Trump’s campaign to force Canada’s annexation as the 51st state through economic chaos.

Prime Minister Mark Carney of Canada was conspicuously silent on Thursday after Mr. Trump signed the executive order implementing tariffs. But the president’s decision to go ahead with higher tariffs on Canada is a blow to the Canadian leader, a political neophyte who was elected to office for the first time just over three months ago.

In June, when Mr. Carney hosted the meeting of leaders of industrialized nations known as the Group of 7, he announced that a trade deal would be reached with Mr. Trump by July 21. And not just any deal. Mr. Carney said that his objective was to eliminate all U.S. tariffs against Canada and return to the free-trade system created by the United States, Mexico and Canada in an agreement Mr. Trump signed during his first term as president.