Intra-Asian defence diplomacy is surging. After laying dormant for decades—thanks to the San Francisco hub-and-spokes system of alliances and regional multilateral dialogues ^[1] which seldom advance actual security cooperation—defence exchanges and strategic relationships are enjoying a new lease of life among Asia–Pacific nations. Indeed, the region has more defence engagement today ^[2] than at any other time in its history.

In December, Australia and Indonesia renewed a defence cooperation agreement ^[3] committing to more military personnel exchanges. Also in December, Shinzo Abe visited India, providing fresh impetus ^[4] to security ties between Tokyo and New Delhi, and Narendra Modi paid a surprise visit to Pakistan ^[5]. Practical defence cooperation is also on the rise: Japan agreed to supply the Philippines with 10 patrol vessels ^[6] for its coast guard and similarly pledged to give Vietnam 6 used patrol vessels ^[7], the first arriving in August last year.

But there’s one security relationship which hasn’t flourished in the way that some might expect: the Japan–South Korea relationship. Despite sharing a mutual ally in the US and similar external threats (North Korea’s nuclear and missile provocations, for example), Tokyo and Seoul haven’t been able to put historical grievances aside and push towards mutually beneficial security cooperation. In all likelihood this relationship will look increasingly anomalous as intra-Asian defence cooperation continues to grow.

Despite what realism says about the huge potential—theoretically at least—for security cooperation between Tokyo and Seoul, the relationship demonstrates the pivotal role history can have in hindering otherwise natural security partnerships. Anti-Japanese sentiment is high in South Korea ^[8] and vice-versa ^[9]—so much so that Japanese cyber nationalists were recently riled up ^[10], airing their anti-Korean sentiments online, over a McDonald’s ad that apparently showed Korean-style bowing.

The major points of conflict that make headlines every now and then include visits to the Yasukuni shrine ^[11] by Japanese prime ministers, the comfort women issue ^[12] and the ongoing territorial dispute ^[13] over the Takeshima/Dokdo islands. The Yasukuni shrine houses a number of ‘class A’ war criminals, who committed crimes against peace, and is seen in South Korea as a symbol of Japan’s historical military aggression; visits by Japanese prime ministers to pay their respects always provokes backlash ^[14] from Seoul.

The comfort women issue is an ongoing one which made its way back into headlines ^[15] late last year when an 80-year old South Korean set himself on fire in a protest calling on Japan to apologise for the comfort women. The two countries released statements ^[16] on the issue in December that (apparently) ‘finally and irreversibly’ resolved the issue. But this hasn’t lasted long and the issue flared up ^[17] again in February.

And while much of the attention on the Asia–Pacific  is currently focused on disputed islands in the South China Sea, Japan and South Korea contest sovereignty ^[18] over a small set of islands in the East Sea—the Takeshima/Dokdo islands. The rocks have been a flashpoint for bilateral disputes in the past: in 2013, South Korean shop owners launched a nationwide boycott of Japanese products ^[19] over the disputed islands and in its 2015 Defence White Paper, the Japanese government reiterated its sovereignty ^[20] (PDF) over the islands which drew criticism ^[21] from Seoul.

Those sore spots wouldn’t cause much concern to strategists if they didn’t affect the overall bilateral security relationship—but they do. In 2012, a military information-sharing agreement was put on hold ^[22] due to domestic backlash in South Korea. Anti-Japanese sentiment proved strong enough to prevent mutually beneficial and strategically important defence cooperation.

The main actor that has previously been able to prompt outbursts of cooperation has been North Korea—a major security concern to both countries ^[23]. In 2014, South Korea, Japan and the US signed a military pact ^[24] agreeing to share intelligence on Pyongyang’s progress towards a nuclear missile. The agreement does not, however, facilitate direct sharing between Japan and South Korea—instead uses the US as an intermediary ^[25]—highlighting the depth of their bilateral grievances.

Following the DPRK’s most recent provocations ^[26], President Barack Obama was able to bring together President Park Geun-hye and Prime Minister Shinzo Abe for a trilateral summit ^[27] to discuss how to manage the North Korean threat. On 18 April, the South Korean Army Chief of Staff visited Japan ^[28] and met with the Japanese Defence Minister to discuss strengthening defence exchanges in light of the North Korean challenge.

Provocation by North Korea combined with US facilitation as a mediator has therefore proven to be the recipe for increased defence diplomacy between Japan and South Korea. But it’s unclear whether Pyongyang’s growing nuclear capability ^[29] will be able to spur meaningful and sustained developments in the security realm—North Korean provocations aren’t the only concern for either country. In all likelihood, the Japan–South Korean relationship will fail to keep pace with the quickening tempo of intra-Asian defence cooperation. And given the potential this relationship has to reshape Northeast Asian security dynamics, that’s a difficult pill to swallow.