OTTAWA — Why is President Trump picking on Canada? Canada has long been the United States’ best customer, buying more American-made goods and services than any other country — more than China, Japan or Britain. American trade with Canada totaled $627.8 billion in 2016; the United States had a $12.5 billion trade surplus. But trade relations have nose-dived since Mr. Trump took office. The president is throwing red meat to his base — much of which feels left behind by the modern economy — by attacking Canada and the North American Free Trade Agreement. Canada is an easy target. In the spring, the Trump administration imposed tariffs costing potentially hundreds of millions of dollars on Canadian softwood-lumber imports, escalating a decades-long dispute between American and Canadian lumber companies. …Mr. Trump also prompted renegotiations of Nafta, which he criticized relentlessly during his campaign as “the worst trade deal ever signed.”