President Trump has not been shy about expressing his attitude toward foreign trade. Throughout the campaign, as well as after assuming office, he has repeatedly argued that it is better to buy American goods rather than imported ones, and has criticized the North American Free Trade Agreement as a bad deal for Americans. In light of this, it is hard not to feel a sense of irony at the latest news from the dairy sector. A new policy from Canada allows the country to block imported dairy products from America, particularly milk that has been heavily filtered. Unsurprisingly, American dairy farmers are crying foul, claiming that the policy violates NAFTA, gives Canada an unfair advantage on the world market, and will cost approximately 10,000 American jobs. They are asking President Trump to intervene on their behalf. Not to sound callous, but I have to ask: What did they expect?When the policy set forth by the president of the United States is one that demonizes trade with other countries, derides our trade agreements as worthless, and conceives of global economics as a zero-sum competition with winners and losers, can you really be surprised when other countries react in the same way? If Trump has no regard for NAFTA, why should Canada? If buying Canadian imports is bad for America, why is buying American imports any better for Canada?This is one of the reasons why protectionism is so dangerous. It not only harms the domestic economy, but it sends a signal to every other country that it’s “us or them” — and if they want to keep up, they had better be at least as aggressive as we are. This is how trade wars start. And if it’s allowed to get out of hand, pretty soon nobody is trading with anybody, with each country becoming its own isolationist island.