Mr. Trump has famously boasted that trade wars are “easy to win,” but he is currently 0-for-2 on that count. Yes, the so-called U.S.M.C.A . treaty has been signed by the United States, Mexico and Canada, but after all the bombast about tearing up Nafta and the insults delivered to America’s allies, Naft2 , as it should be called, is a conventional, incremental result that isn’t markedly different from the original. “The economic implications are little to none,” noted UBS Wealth Management’s chief economist, Paul Donovan. And there’s no guarantee that Congress will approve Naft2 or that Mexico’s new president, Andrés Manuel López Obrador, will abide by it.

Trade wars tend to be easier to prolong than they are to win or even end. And the current one is hurting both the United States and China, as well as rattling around like a loose part in the global economic machine. Both nations are heading for economic slowdowns. Growth in the United States will fall to 2.1 percent in 2019 from 2.9 percent this year, according to the bank BNP Paribas. The United States is slowing because the stimulus from last year’s tax cuts won’t be around t his January to juice the stock market or consumer spending. China’s economy is cycling down just as its export machine is facing lower aggregate demand, a combination that could trim growth to 6.2 percent from 6.6 percent, BNP Paribas says. That’s still fairly muscular compared with the rest of the world, but Mr. Xi doesn’t want China’s economic miracle to sputter under his watch.

Whatever the rationale for imposing them, tariffs are taxes, and consumers were facing a big hike in the Trump tax in January. Meanwhile, the president has little to show for his trade war. He has been obsessed with reducing the trade deficit, which stood at $566 billion last year. Yet the trade deficit is rising, not falling, because Americans like buying inexpensive goods from foreign nations (like, say, China). And the dollar has strengthened, which generally makes imports cheaper. Nor have jobs come flooding back to the United States. Instead, tariffs prompted some American companies to shift their sourcing of goods from China to other parts of Asia. Those goods will still be imported. At the same time, for companies such as General Motors and Caterpillar that rely on Chinese parts or metals, tariffs have added hundreds of millions of dollars of costs. That’s one of several reasons GM is cutting jobs at some assembly plants in the Midwest. The agriculture sector, which counts China as a major customer, has been hard hit by the loss in exports.

The uncertainty created by Mr. Trump’s threats of a trade war escalation has been even more damaging. Businesses and investors can’t abide uncertainty, and that contributed to Wall Street’s November swoon, which wiped out all of the year’s gains at one point. They wanted the saber rattling to stop; when the truce was announced, the market rallied.