American diplomats like to portray their country’s allies in glowing terms. So the world should take note when they do not ― such as when U.S. Undersecretary of State Wendy Sherman, at a recent conference in Washington, D.C., on Asian security, publicly scolded South Korea for its seemingly endless vilification of Japan. According to Sherman, South Korea’s stance ― reflected in its demand that Japan apologize, once again, for forcing Korean women to provide sexual services to the Imperial Army during World War II ― has produced “paralysis, not progress.” 

But Sherman’s criticism could also be leveled against Japan. Prime Minister Shinzo Abe has rarely missed an opportunity to provoke Japan’s Korean critics, whether by visiting Tokyo’s Yasukuni shrine, where the “souls” of 14 Class A war criminals are interred, or embracing revisionist critiques of previous official apologies for Japanese aggression.

Instead of working together to help their American ally confront the challenges posed by a rising China and the North Korean nuclear threat, South Korea and Japan have allowed their rancor to stymie effective action. This seemingly endless tension has been frustrating ― and worrying ― American leaders for years, especially as it has undermined the United States’ strategic “pivot” toward Asia.

Since President Barack Obama announced the pivot five years ago, the U.S. has been attempting to bolster its forces and alliances in Asia, thereby reinforcing its strategic role in a region that China is increasingly attempting to dominate. But the relentless sniping by its two most important Asian allies has blocked the kind of concrete cooperation needed to help it achieve its main goals, including ensuring a durable, long-term military presence in the region.

Intelligence sharing is a case in point. In December, U.S. officials, seeking to better their understanding of North Korea’s missile and nuclear programs ― and enable commanders to react swiftly if potential threats materialize ― announced a new information-sharing agreement with South Korea and Japan. But the deal could be a script for a situation comedy: Japan and South Korea refuse to provide intelligence data to each other, leaving the U.S. to play the middleman.

The U.S. has accentuated the positive, calling the agreement an important step forward. But, though it does represent progress from 2012, when popular opposition in South Korea to the idea of military cooperation with Japan caused a similar agreement to collapse, the latest effort is inefficient, at best.

China has been eager to capitalize on the animosity between Japan and South Korea to undermine America’s security interests in Asia. During a visit to South Korea last July, President Xi Jinping highlighted not only the two countries’ deepening economic relationship, but also their shared views regarding Japan’s wartime past. Other Chinese officials have picked up the theme, dropping hints that China’s 70th anniversary celebration of the end of World War II could exclude Japan ― unless, that is, Japan is more contrite about its historic transgressions.

It is time for the U.S. to tell its Asian allies to get over it. As the underwriter of both Japan’s and South Korea’s national defense, the U.S. simply cannot allow their historical animosities to impede action to address urgent threats in this critical region.

The timing could not be better, as rising security fears are altering public perceptions. Recent opinion polls suggest that at least half of all South Koreans are worried enough about regional tensions to support closer military ties with Japan.

And, indeed, the security risks facing Asia are only growing ― exemplified in China’s move from military expansion to blatant assertiveness. Most notably, in the South and East China Seas, China has been staking its claim to disputed island territories, deploying advanced military hardware, and aggressively patrolling an expanded security zone. Meanwhile, leaks from Chinese think tanks have suggested that if the North Korean regime collapses, China could well send troops to preserve the country’s stability.

Asia’s new security landscape places a premium on seamless cooperation among U.S. allies ― a prospect that the sustained bickering between South Korea and Japan calls into question. It could even be said that their longstanding dispute makes America’s Asian alliance system worth less than the sum of its parts.

Repairing relations between South Korean and Japan could not be more urgent. Even with good will on both sides, it will take time to build a strong defense partnership. Effective military cooperation requires personal ties that take years to build, and, aside from some joint naval and air exercises, the two countries have little experience working together. Boosting technical interoperability also will take considerable time, though both countries maintain sophisticated defense forces with great potential to be linked together.

Effective cooperation will also require a broader scope for joint action ― an imperative that is not reflected in the recent intelligence-sharing agreement. The risks to stability in Northeast Asia extend well beyond North Korea’s nuclear weapons and missile programs, and a joint intelligence agenda should address them. How will the allies respond to the threat of a conventional military attack by ― or instability in ― North Korea? What if the North Korean regime collapses, and China does intervene militarily?

After spending the last six decades defending South Korea and Japan, the U.S. has every reason ― and plenty of leverage ― to demand that its two long-time allies enhance their military cooperation. Simply focusing on the positive ― America’s classic approach to alliance diplomacy ― is no longer enough.

Whatever their historical disagreements, South Korea and Japan both face serious risks in their immediate neighborhood. It is up to the U.S. to ensure that they work together to protect themselves ― and ensure long-term stability throughout Asia.

Kent Harrington, a former senior CIA analyst, was a national intelligence officer for East Asia, chief of station in Asia, and served as the CIA’s director of public affairs. ― Ed.

(Project Syndicate)