Why would we make Canada’s scuffling economic performance our problem? The slogan of James K. Polk’s supporters in the 1844 presidential campaign was “54° 40′ or Fight,” referring to their desire to take a substantial slice of what would eventually become Canadian territory along the Pacific Northwest coastline. For his part, Donald Trump isn’t bothering to set out any latitude lines when he discusses his ambitions on Canada — he wants the whole thing, or so he says in his trolling...