Yes, the world would be a better place if there were no trade barriers at all. But the United States should abolish its barriers even if China, Canada or others do not abolish theirs. As the economist and Times Op-Ed columnist Paul Krugman explained in an academic journal, “The economist’s case for free trade is essentially a unilateral case: A country serves its own interests by pursuing free trade regardless of what other countries may do.”

Mr. Krugman also lamented that economists “must deal with a world that does not understand or accept that case” for unilateral free trade. That becomes clearer by the day: Now so-called free traders seem to believe that the president’s $50 billion of tariffs on imports from China is part of a brilliant strategy to get the Chinese to open their markets to American exports. Similarly, they applaud Mr. Trump for starting a trade war with G-7 countries toward the goal of a tariff-free G-7.

Even if Canada never removes its 270 percent tariffs on our dairy products, Americans would gain if Uncle Sam, regardless of Ottawa’s trade policies, unilaterally removed not only the steel and aluminum tariffs it just slapped on Americans who buy Canadian metal but also ended all tariffs on imports from Canada.

Don’t forget that Canada’s dairy tariffs are paid by Canadian consumers. It defies logic for an American president to punish American consumers in order to prompt Justin Trudeau to be kinder to Canadians. We also know that an increase in imports from Canada will expand our exports to our northern neighbor. By contrast, protectionist policies like those supported by the Trump administration may lead to more, not fewer, protectionist policies abroad — tariff hikes that have been historically ineffective.

Finally, the case for low trade barriers is not simply theoretical. It’s currently on display in several places in the world, including Singapore, the Netherlands and New Zealand. Hong Kong is a case worth highlighting. Thanks to its history of free trade under British rule and current special status in China, it’s widely regarded as one of the least restrictive economies in the world. Among the policies that have fueled its growth is unilateral free trade.

Far from suffering from its free-trade stance, Hong Kong’s economy has experienced multiple periods of rapid growth. In 1950 its average per capita income was about one-third the average United States income, but by 2017 it was slightly higher. In 1960 life expectancy in Hong Kong was three years lower than in the United States, whereas by 2017 it was five years longer. Sure, other free-market policies contributed to this economic success story, but at the very least unilateral free trade hasn’t stopped Hong Kong’s transformation into one of the richest economies in the world.

President Trump should take a page from Hong Kong. As that territory’s experience demonstrates, and as economists have long argued, lowering trade barriers regardless of other governments’ trade policies fuels domestic economic growth. So if Mr. Trump insists on acting unilaterally, he should cut rather than raise tariffs.