President Donald Trump’s tough line on trade with China has finally given Japanese Prime Minister Shinzo Abe something to agree with Beijing about.

After years of skirmishing with China over territory and security, Mr. Abe is promoting a rapprochement that reflects Tokyo’s need for allies in upholding the postwar free-trading system. It’s an example of how Mr. Trump’s “America first” policy of curbing imports he says threaten the U.S. are reshaping relations around the globe.

“I want to lift up the Japan-China relationship to a new stage,” Mr. Abe said at a news conference this month, describing a May visit to Japan by Chinese Premier Li Keqiang as “an important first step toward a dramatic improvement.”

During his visit, Mr. Li highlighted how Japan complements China as an export powerhouse, saying this results in “strong competitiveness in third-country markets” such as the U.S. He barely mentioned the historical issues that divide the two countries—Japan occupied much of China after invasions in the 1930s—and he repeated Tokyo’s favored formulation that the countries should “look to the future.”

Japan exported some $137 billion of goods to China in the most recent fiscal year, much of it semiconductors and other high-tech electronics used by Chinese factories to make products such as iPhones destined for the U.S. That’s why Mr. Trump’s initial $50 billion in tariffs on Chinese products, focusing on high-tech items, could end up hitting Japan as well.

The tariffs, and China’s vow to retaliate, led the Nikkei Stock Average to fall 0.75% Monday.

At a widely noted speech last November, Mr. Xi said, “We should uphold multilateralism” and “forge closer partnerships.”

Beyond that, as China’s own economy slows, Beijing is looking more toward Japan for investment and expertise in advanced manufacturing. Messrs. Xi and Abe spoke by phone for the first time in May and agreed it was an opportune time to improve relations. During the call, Mr. Xi said that “for a period of time, Japan has released positive signals and taken positive actions on bilateral relations,” according to official media.