But the rest of the planet is increasingly refusing to play along, instead seeking alternatives to trade with a suddenly mercurial United States. This year, Europe and Japan set in motion a mammoth trade deal that generates fresh opportunities for their companies, while leaving American players at a disadvantage.

Under the Trans-Pacific Partnership, a separate trade deal forged by 11 countries, Japan agreed to open its heavily protected market to agricultural imports, handing American farmers a lucrative opportunity. Days into his presidency, Mr. Trump closed that window by renouncing American participation in the deal. Now, European farmers have secured their own broadened access to the Japanese market.

China has responded to Mr. Trump’s tariffs with levies on American goods, including soybeans, wood products, and machinery. At the same time, China has lowered tariffs on imports from countries like Germany and Canada, lifting the fortunes of American competitors in those nations, according to an analysis by Chad P. Bown at the Peterson Institute.

For example, China doubled duties on American agricultural and fish products to an average of 42 percent from 21 percent, Mr. Bown found. At the same time, China lowered average tariffs on the same wares from the rest of the world to 19 percent.

Mr. Trump has sold his trade war as a means of returning jobs to a forsaken American heartland. In his version, naïve presidents allowed other nations, especially China, to steal factory jobs from the United States. His tariffs will pressure multinational companies to bring those jobs back.

But if that story makes for rewarding politics, it rests on antiquated economic assumptions about the global marketplace.

Mr. Trump’s trade war will probably not increase the number of American jobs, although it has imperiled paychecks at auto plants and other factories that rely on imported components. What it will bring is higher prices for manufactured goods, economists say, along with uncertainty over the terms of trade. It is sapping vitality from an already weakening world economy.