PRESIDENT TRUMP has been fixated on Japan’s alleged exploitation of the United States for decades. Starting in the mid-1980s, he criticized Japan for protecting its markets unfairly against U.S. products while this country helped protect it from the Soviet Union and other threats. He took out a newspaper advertisement in 1987, claiming that “for decades, Japan and others have been taking advantage of the United States,” as a New York Times report reminds us. As historian Jennifer M. Miller of Dartmouth College has argued in a study, the origin of Mr. Trump’s entire outlook on foreign policy and economics may be attributed in large part to his response to Japan’s rise from the ashes of World War II. His grievances against Japan, and his belief that tariffs could remedy them, amount to a template that he has applied, as president, to countries as different as China and Germany.

And so there is a special significance in the fact that Mr. Trump has just concluded a new trade agreement with Prime Minister Shinzo Abe of Japan. Consider it a test of what happened when the erstwhile New York real estate developer finally got to act on his long-standing theory. Answer: not much. After threatening Japan with tariffs on auto exports to the United States — the largest category of Japanese goods, worth $51 billion in 2018 — Mr. Trump extracted from Mr. Abe a promise (subject to parliamentary approval) to give U.S. beef, pork and other agricultural goods the same improved access the Obama administration had negotiated in the Trans-Pacific Partnership deal. In return, Mr. Trump essentially gave Mr. Abe his word of honor that he won’t impose tariffs or import quotas while the two sides continue wider talks.

Mr. Abe must be a trusting man. He has to be, given Japan’s dependence on the United States for defense and other support. Still, he also can claim a victory for his own strategy of joining a deal with other Pacific Rim nations, equivalent to the TPP, that excluded the United States, and thus made it more difficult for U.S. agricultural producers to compete for access to Japan’s notoriously closed market.

In short, the episode shows how little was gained from Mr. Trump’s decision to go it alone in trade, as opposed to continuing with a broad multilateral approach that would have accrued allies — Japan included — to work together against the source of their most justified concerns, China. Even if you believe the U.S. merchandise trade deficit with Japan is a problem, it is far smaller than it was when Mr. Trump first noticed it: $67.6 billion, out of a global deficit of $891 billion in 2018, as opposed to $46.1 billion out of $148.5 billion in 1985. (Then, as now, the deficit is smaller when services are included.)

The Trump administration estimates that its deal with Mr. Abe could boost market access in Japan for $7.2 billion worth of U.S. farm products. Undoubtedly, this will gladden some hearts in rural areas hit both by the trade tiff with Japan and by the wider tariff battle with China. Everyone should remember, though, that whatever damage is partially undone was damage Mr. Trump partly caused himself.

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