President Donald Trump’s tariffs on Canadian goods, coming on the heels of his taunting threats to make the country the 51st U.S. state, are eroding the inherent politeness of Americans’ northern neighbors and rallying them around their own flag. Canadians are removing American liquor and California wines from their store shelves. They’re pulling back on future visits to the U.S. They’re pushing “Buy Canadian” to counter higher costs and spite Trump. And they are uncharacteristically brandishing unvarnished anger over what they see as a betrayal by a longtime friend. “I will never visit America again,” Angela Qin, a university student,...