THE CONFIDENCE, even a certain complacency, that had developed around the likelihood of a successful conclusion to trade talks between China and the United States has given way to tension and uncertainty. On Sunday, President Trump, blaming Beijing for reneging on previously negotiated commitments, tweeted a threat to the effect that he would raise an existing 10 percent tariff on $200 billion worth of Chinese imports to 25 percent on Friday, with 25 percent tariffs on additional Chinese goods coming “shortly.”

News reports identify the Trump administration’s specific complaint as China’s alleged going back on its promises to put U.S.-requested policy changes into law. For their part, Chinese sources have told Western media that Beijing interpreted Mr. Trump’s complaints about purportedly tight Federal Reserve monetary policy as a sign of economic weakness that China could exploit.

It’s anyone guess what will come of the current meetings in Washington between China’s trade delegation and Mr. Trump’s team. What should not be in doubt, however, is that throughout the entire bargaining process with Beijing, the administration has undercut its position by attempting to wage simultaneous tariff battles with other countries. As Chad Bown of the Peterson Institute for International Economics has shown, Mr. Trump’s tariffs now cover 50.5 percent of Chinese imports — but also 7.3 percent of imports from Canada, 2.5 percent of imports from the European Union, 9.6 percent of imports from South Korea and 3.8 percent of imports from Japan.

Though still modest in their economic impact, the tariffs have symbolism that stings these long-standing U.S. allies and trading partners. For the most part, they do not abuse international trade rules as China does. Indeed, they share many of the United States’ concerns with Chinese protectionism, theft of intellectual property and subsidized state enterprises. Yet by bracketing old friends with Beijing, even justifying some levies against them (on steel and aluminum) on “national security” grounds, Mr. Trump has made it politically difficult for them to rally to the U.S. side in the dispute with China. They were in fact obliged to retaliate.

This unforced error is doubly regrettable because Mr. Trump arguably had the upper hand going into his talks with China. The Chinese economy is more dependent on exports to the United States than vice-versa, and China’s growth rate has been slowing, while the U.S. economy (contrary to Mr. Trump’s inappropriate and ill-founded gripes about Fed policy) is creating jobs at a brisk pace. In a world where Mr. Trump is deeply unpopular, his complaints — shared by previous U.S. presidents — against China represented a rare case in which other nations conceded him the moral high ground. A savvier president would take advantage of that.

The negotiations with China may yet reach a mutually beneficial conclusion, or they may collapse. Either way, Mr. Trump will have defied a lesson of history: Multiple-front wars are the hardest to win, whether they are of the military kind or trade wars. Fighting without allies is harder still.

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