President Trump says that Canada poses a national security threat to the United States. He has therefore imposed tariffs of 25 percent on imported Canadian steel products and 10 percent on Canadian aluminum products.

Canadian Prime Minister Justin Trudeau reacted by pointing out that "our soldiers who had fought and died together on the beaches of World War II and the mountains of Afghanistan, and have stood shoulder to shoulder in some of the most difficult places in the world, that are always there for each other, somehow - this is insulting to them" - but his recollection of all that is of no consequence. Neither is the prime minister's puzzlement that "the idea that the Canadian steel that's in military - military - vehicles in the United States, the Canadian aluminum that makes your, your fighter jets, is somehow now a threat."

If President Trump says Canada is a threat, it must be true, because he says so.

Canada indeed has been a threat in what, for the president, apparently is the recent past, namely, the 19th century. After all, it was not that long ago that British Canada was at war with the United States. Indeed, it was in September 1813 that Oliver Hazard Perry defeated a British naval force on Lake Erie, one of the great American naval triumphs of the War of 1812.

Some 30 years later, the United States almost went to war with Britain over the Oregon Territory-Canadian boundary - hence, the famous American rallying cry, "54-40 or fight." Finally, President James Buchanan apparently provided the most immediate model for our current president by supporting the 1861 Morrill tariff on goods (including those from Canada) that were imported into the United States. Buchanan sought to protect fledgling American industries; evidently Mr. Trump feels those industries still are not fully mature and need protection. And while Canada may no longer be under direct British rule, it still poses a national security threat to the United States, at least in his mind.

All of the foregoing no doubt will come as a surprise to Lt. Gen. Pierre St-Amand, the Canadian officer who happens to be deputy commander of the North American Aerospace Command, located at Peterson Air Force Base in Colorado Springs, Colorado, with Cheyenne Mountain as its alternate command center.

St-Amand and his fellow Canadian military personnel, who jointly man that command with American forces, are responsible for collecting and coordinating the worldwide system of sensors that provide both American and Canadian National Command authorities with early warning of air, space or maritime threats to both countries. Evidently, according to the president, Gen. St-Amand and his Canadian subordinates no longer can be trusted to provide such warning, since they embody a resuscitated Canadian threat to America's national security.

Nonsensical as the president's explanation for the tariff on Canadian goods might be, it has provoked a very serious reaction from Ottawa: Canada will impose its own tariffs on nearly $13 billion worth of American goods, including beer, whiskey and yogurt. Of the six states hardest hit by the Canadian tariffs - Michigan, Ohio, Pennsylvania, New York and Illinois, with more than $1 billion dollars in exports to Canada, and Texas, with about $900 million - all but New York and Illinois went for Trump in the 2016 election.

Hopefully, the Canadian reaction will be limited to trade, and Trudeau's riposte to Trump's national-security crack will be the final word on that matter. Neither country can afford tension within the NORAD command, much less its dissolution; the only beneficiaries would be Russia and China. But it is hard to see the command remaining intact if half its personnel continue to be treated as a threat to the other half because they wear the maple-leaf uniform.

Mr. Trump would be wise to let the matter drop and save his tweets and commentary for other matters that happen to cross his mind.

Dov S. Zakheim is a senior advisor at the Center for Strategic and International Studies and vice chairman of the board for the Foreign Policy Research Institute. He was under secretary of Defense (comptroller) and chief financial officer for the Department of Defense from 2001 to 2004 and a deputy under secretary of Defense from 1985 to 1987.