Japan and South Korea are mired in a heated military spat over an encounter at sea last month between a South Korean warship and a Japanese maritime patrol plane. Tokyo claims that its aircraft was threatened by the South Korean ship’s targeting radar for surface-to-air weapons, a charge that Seoul flatly denies. Instead, it accuses the Japanese military of provocatively flying its planes at low altitudes. 

The escalating feud is further straining an already tense bilateral relationship, as the two sides struggle to resolve difficult historical issues over Japan’s colonial occupation of Korea that have resurfaced in recent months. The United States, which has a history of stepping in to mediate between its two allies, appears to have made no such efforts so far in this case. If left unresolved, the latest row threatens to not only undermine military cooperation between Japan and South Korea, but also coordination on addressing the North Korean nuclear threat.

On Dec. 20, a Japanese P-1 maritime patrol aircraft was conducting a surveillance operation over the Sea of Japan. It was observing and documenting the activities of two South Korean warships operating in international waters within Japan’s exclusive economic zone. One of the South Korean vessels was a coast guard patrol craft aiding a North Korean fishing boat in distress; the other was a Republic of Korea Navy destroyer.
In a video of the encounter subsequently released by the Japanese Ministry of Defense, the P-1’s crew can be heard taking note of routine details of the scene, such as the distance between the ships, their hull numbers, whether a helicopter could be observed on the destroyer’s helipad, and the prevailing weather conditions. But the mood on the plane turns tense when an airman detects fire-control radar, which is used to guide missiles and other munitions to their intended targets, from the South Korean destroyer. Successive attempts to establish radio contact with the ship go unanswered.
In the ensuing days, the Japanese government complained repeatedly and asked the South Korean military to prevent another such incident, noting that the use of fire-control radar can be interpreted as a threat and could lead to a dangerous chain reaction. Rejecting the allegations, Seoul claimed that while its destroyer was using radar as part of a search-and-rescue operation, it was never directed at the Japanese plane, and radio calls to the ship were incomprehensible due to static.

The South Korean Ministry of National Defense has also demanded—most prominently in a video of its own released to counter the Japanese one—that Tokyo apologize for its “unmannerly” and “threatening” low-altitude flight at the site of a humanitarian rescue operation. Over the course of several rounds of working-level diplomatic talks, the two sides were unable to reach even a shared understanding on basic details of what transpired that day. 

Finally, Japan’s Ministry of Defense announced last week that it was breaking off consultations on the matter, shortly after releasing new audio of what it claims are sounds from the South Korean destroyer’s fire-control radar system. The South Korean government dismissed the recording as “merely mechanical sounds” that don’t amount to evidence. 

Military tensions between Japan and South Korea add to existing strains in the relationship over deep-rooted historical issues.A meeting last Wednesday at the World Economic Forum in Davos, between Japanese Foreign Minister Taro Kono and his South Korean counterpart, Kang Kyung-wha, provided an opportunity to put a floor under the spiraling diplomatic crisis. But it merely illustrated the depth of the rift, as the two ministers traded accusations in full view of the assembled press. Earlier that day, the South Korean military had upped the ante, threatening to “react resolutely according to the response protocol of the [South Korean] military” in response to Japan’s “provocative acts.” 
So far, the war of words remains just that, and chances of actual shots being fired are slim. Still, there are good reasons for regional observers to worry. For one thing, direct involvement by the Japanese and South Korean militaries in such a high-profile political spat is rare. Even during the most acrimonious debates of years past, the two countries’ defense establishments could be relied upon to act professionally and apolitically based on a shared understanding of the common threats that they face in the region. In other words, security ties were to a certain extent compartmentalized from the political sphere. But now, the militaries have been dragged down into the mud.

The tensions add to existing strains in the relationship over deep-rooted historical issues. Recent rulings by South Korea’s Supreme Court have found Japanese companies liable for compensation toward Koreans who were forced to work without pay in Japanese mines, factories and mills during World War II. Earlier this month, a court in the city of Daegu ordered the seizure of local assets held by one of the Japanese companies, Nippon Steel and Sumitomo Metal Corp. That prompted a request from Tokyo for direct talks with the South Korean government over the issue, but President Moon Jae-in’s administration has yet to respond. 
These parallel crises are already hurting bilateral security ties. Japan’s Sankei Shimbun newspaper reported over the weekend that Tokyo plans to shrink the scope of its defense cooperation with South Korea, first by canceling plans to send a helicopter carrier to South Korea in April. A Japanese official told Sankei that a “cooling-off period” is needed. The tensions have also resulted in at least one canceled personnel exchange, and they also threaten to disrupt intelligence-sharing. 

Given the United States’ strategic interest in ensuring effective trilateral cooperation with South Korea and Japan, it is reasonable to ask why Washington has yet to take an active interest in breaking the logjam. The White House would understandably prefer Seoul and Tokyo to work things out on their own, but it has shown a willingness in the past to engage its Northeast Asian allies at a high level. In 2014, then-President Barack Obama brought the Japanese and South Korean leaders together for a face-to-face meeting to discuss threats emanating from North Korea, setting the tone for a gradual thaw over the course of the next two years. Unfortunately, the Trump administration—understaffed, preoccupied with other matters and blindered by its tendency to view foreign policy through a bilateral lens—has shown no such inclination. 

The problem is further amplified by heightened nationalist sentiments on both sides. A recent public opinion poll in Japan found that 62 percent of respondents believe stronger measures should be taken against Seoul. Meanwhile, as part of their ongoing rapprochement, the two Koreas are preparing to jointly commemorate the centennial of the founding of the Korean independence movement against Japanese colonial rule, known in Korea as the March 1 Movement. 

Pyongyang can only benefit from this discord. It has already seized on it to promote a picture of a unified Korean stance against Japan’s modern-day intransigence and military provocations, in an attempt to cast Japan as the primary regional threat, rather than North Korea’s own nuclear program. North Korean state media has been full of aggressive anti-Japanese commentary recently, with Tokyo largely supplanting Washington as the main bogeyman. 

In that context, the upcoming March 1 Movement centennial provides an opportunity for Seoul to further the cause of inter-Korean unity by stoking anti-Japanese sentiment. For Moon, who has largely staked his political legacy on a policy of engagement and improved relations with North Korea, that opportunity may prove difficult to turn down. To the extent that he decides to take it up, however, it will introduce new volatility into U.S.-led efforts to persuade North Korea to abandon its nuclear program.