[Daniel DePetris] Trump’s coercion has limits

As if US officials didn’t have enough on their plate these days, they can now add a trade war with Canada to the mix — and with it, one more dent to a relationship that historically is one of the closest the United States has with another country.

President Donald Trump has done the impossible by turning America’s pleasant neighbor up north into a combatant.

After Trump in July threatened to enact an additional 50 percent tariff on $20 billion worth of Canadian exports to the United States, US a