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The Obama administration excels at annoying U.S. allies. A senior official’s recent labeling of Israel’s Prime Minister Benjamin Netanyahu as a form of avian excrement was only the latest misstep. Besides aggravating Israel and Saudi Arabia over how to check Iran’s Middle East ambitions, Canada has been left waiting for years on the Keystone XL pipeline, and Britain still remembers the 2012 debate over whether a bust of Winston Churchill belonged in the Oval Office.

This week’s Asia-Pacific Economic Cooperation summit in Beijing presents President Barack Obama with an opportunity to break that cycle by bolstering his relationship with Japan’s Prime Minister Shinzo Abe. Despite differing political perspectives—Mr. Abe sits right of center while Mr. Obama leans to the left—and Mr. Abe’s slumping popularity at home, the prime minister has put his reputation on the line to strengthen the U.S.-Japan alliance. Were Mr. Obama to meet him in the middle, Japan could become an important key to the future modernization of the U.S. military and the security of East Asia.

To Mr. Obama’s credit, he had some reassuring words to say about Mr. Abe and Japan during a joint press conference in April. The president thanked Mr. Abe “for your friendship, your partnership, and the progress we’ve made together�� on economic issues as well as on regional security as a result of Mr. Abe’s new, if controversial, emphasis on Japan’s right to collective self-defense.

The Obama administration has also made it clear that it will not let China bully Japan over the Senkaku Islands, with the president saying that Japan’s administration of the islands is “a consistent part of the alliance�� between the U.S. and Japan.

Still, there will be those at the APEC meeting who would like to put some distance between Messrs. Obama and Abe. One of them is South Korea’s President Park Geun-hye. She’s been furious with Mr. Abe ever since his controversial visit to the Yasukuni Shrine for Japanese war dead, some of whom South Koreans consider war criminals. Ms. Park also attacked Mr. Abe on the issue of Korean “comfort women,�� who were forced to service Japanese soldiers during World War II. Ms. Park and her compatriots are seeking a profuse public apology from Mr. Abe.

Likewise, China’s Supreme Leader Xi Jinping would be delighted to see the rift between South Korea and Japan widen. Mr. Xi and the Chinese media have relentlessly emphasized the comfort-women issue. At the APEC summit Mr. Xi will no doubt encourage Mr. Obama to take a more “even handed�� approach to the Senkaku dispute. He will also hope that Mr. Obama will distance himself from Mr. Abe’s new defense policy if he suggests it could injure relations between Beijing and Washington, as well as those between Beijing and Tokyo.

Mr. Obama will also be aware of Mr. Abe’s steady drop in popularity at home. “Abenomics�� has stalled, and Mr. Abe’s policies on defense and closer military cooperation with the U.S. has come under attack from the pacifist Japanese left.

But Japan is exactly the kind of ally the U.S. needs in the region. With more naval vessels than France and an army larger than Germany’s, Japan can no longer shrug off its share of the military burden in the U.S.-Japan alliance. At the same time, Japan is going to need American help in developing and deploying advanced technology for protecting Japan’s homeland, from antiballistic missile defense systems to unmanned arial vehicles.

And here’s where Mr. Abe’s recent lifting of a decades-old ban on Japanese defense exports becomes crucial. The ban’s end isn’t just good news for Japanese defense firms or for countries like India and Australia, who want to buy advanced submarines and seaplanes from Japan—it’s also good news for the U.S. It raises the possibility of joint ventures between the world’s two most sophisticated high-tech economies in developing future defense systems, from space and cyber defense to robotics and high-end electronic warfare.

Last year, a Japanese company won highest honors at the Robotics Challenge sponsored by the Pentagon’s Defense Advanced Research Projects Agency. It’s a harbinger of the military transformation that could arise if Japan continues on the path Mr. Abe has taken—and could counter worries recently expressed by a senior Pentagon official that the U.S. is losing its long-standing military-technology edge to China.

Mr. Abe’s steadfast stand on Japan’s defense doesn’t just bode well for his country’s future or the health of the U.S.-Japan alliance. It’s also good for America’s own future security. Mr. Obama needs to show his support for Mr. Abe at APEC. He must prove that this alliance is built to last.