It may be his trickiest mission yet. US Joint Chiefs of Staff Chairman Mark Milley is a dangerous-looking hunk of bone and gristle. Sternness marks his craggy mug; Special Forces and Ranger tabs decorate his uniformed shoulders; and under his bemedaled chest beats, no doubt, the heart of a boot-wearing, steak-eating badass.

But it will not be martial skills that Milley needs on his visit to South Korea. After a flying visit to Japan, he arrived in South Korea on Wednesday, and will be joined on Thursday by US Secretary of Defense Mark Esper.

It’s a safe bet that their visit will be marked by hand-holding photo-ops and predictable PR blather (“The alliance has never been stronger,” etc, etc). But behind closed doors, it is to be hoped that both men have packed their diplomatic A-games.

South Korea is holding its breath, for it knows the duo’s mission.

Vexing issues

The mechanics of the transfer of wartime operational control from US to South Korean command, expected around 2022, will hover in the background. In the foreground, the Americans want Seoul to cough up more wonga for the 28,500 US troops stationed in South Korea.

Above all, they want Seoul to reverse its decision to nix an intelligence-sharing arrangement with Japan that Seoul has made clear will expire on November 23. Seoul, however, has recently stated that it will not do a U-turn.

The pact, the GSOMIA (General Sharing of Military Information Agreement), is the only bilateral, security-related agreement linking two nations that are often – wrongly – dubbed “allies.”

True, both nations boast bilateral alliances with the US. True, they are both democracies that share related values and cultures. True, they are economically closely interlinked. And true, they both face off against a rising China and a dangerous North Korea.

But they are convulsed by historical and historiographical disputes that – in South Korea at least – may trump all considerations listed prior. Korea’s national gut is deeply roiled by anti-Japanese sentiment, dating back to 1910-45, when Japan colonized the Korean Peninsula.

The Seoul-Tokyo non-alliance

There is no question that it was an exploitative and often brutal rule. Japan invested in its colony for its own economic benefits. In the twilight years of that era, it suppressed native culture. It also mobilized countless Koreans into military service, forced labor and, most infamously, military brothels.

Today, Koreans rightly accuse Japanese school textbooks of whitewashing history, and point fingers at revisionist figures in the political class who play down Japanese aggression or – as per the narrative at the infamous Yasukuni Shrine Museum – paint Japan as a victim. They also accuse Japan of not apologizing sincerely, of undercutting its own apologies, and of not being as fulsome with its apologia as Germany.

Still, Germany committed genocide. It is difficult to point to a former metropole on the global scene that has said sorry more, or offered more compensation to a former colony, than Japan has to South Korea.

Japanese officials – from emperors, prime ministers and cabinet secretaries on down – have apologized scores of times. Tokyo has granted hundreds of millions of dollars in compensation packages. And in the public domain (if not textbooks), there is plentiful information, and debate, about Pacific War-era atrocities.

Most problematically – and perhaps puzzlingly – for Washington, South Korean sensitivities look lopsided given the lack of fury aimed over the Demilitarized Zone.

Why no North Korea hate?

Japan’s 35-year rule was less destructive (in terms of both life and property) than the 1950-53 Korean War initiated by Pyongyang. North Korea today is an isolated, family-run, nuclear-armed dictatorship that suppresses millions of Koreans; Japan is an open democracy with constitutionally trammeled armed forces that oppress nobody.

But though the dictatorship in Pyongyang has proved longer-lived than Japanese colonialism, and North Korea has neither apologized for nor compensated for the war, South Koreans despise Japan more than their brother nation up North.

A poll for a state-run think-tank last week discovered that, were war to break out between Pyongyang and Tokyo, 45.5% of South Koreans would help North Korea, and only 15.1% Japan.

In sync with these sentiments, the left-leaning Moon Jae-in administration has focused on trying to upgrade ties with North Korea while firing relentless salvos against Japan.

Last year, the South Korean state unilaterally undercut two bilateral deals (both packaged with related compensation): a 2015 agreement on “comfort women” and a 1965 agreement on forced labor.

In response, in July this year, Tokyo introduced trade curbs on exports to South Korea – slowing, but not halting, the flow of key materials.

That infuriated South Korea. Citizens reacted with consumer boycotts, while Seoul retaliated with its own trade curbs. Seoul then punted the dispute from the historical, diplomatic and economic spheres into the security space, announcing it was ending GSOMIA.

The surprise move prompted Washington to aim some undiplomatic language against Seoul. But Seoul was still not finished with Tokyo.

It complained to the International Olympic Committee that Tokyo would not ban the “Rising Sun” flag at the 2020 Olympics, while a new lawsuit brought by comfort women against Tokyo kicked off on Wednesday in a Seoul court.

The ever-widening rift between the capitals represents a major headache for the United States, which seeks to forge a united front in Northeast Asia.

Yet however much Seoul’s bottomless fount of anti-Japaneseism exasperates them, Milley and Esper would be well advised to restrain their voices and not hammer too loudly upon the negotiating table.

Why South Korea matters

First, South Korea is, geo-strategically, part of a critical perimeter for the United States – the frontline of a deeply echeloned Pacific defense that is layered back through Japan, Guam and Hawaii before reaching the continental US itself.

At the epicenter of Northeast Asia, South Korea is also well sited as a launch pad to monitor and interdict the assets of regional American competitors such as China, North Korea and Russia.

And the Moon administration has kept faith with its overthrown predecessor, the Park Geun-hye administration, by maintaining the US Terminal High Altitude Area Defense (THAAD) anti-missile system on Korean soil. That decision has come at massive national cost.

A furious China, which asserts that THAAD radars can snoop on its own territory, has banned K-pop concerts and refuses to permit Korean online games. Korean corporations, notably Lotte Group, have taken a drubbing in the market.

Looking further back, South Korean soldiers have done what Japanese troops never have: Shed blood alongside GIs. After its joint effort with Washington in the Korean War, Seoul dispatched the largest national contingent to fight for South Vietnam. It subsequently deployed a major contingent to northern Iraq.

Most recently, South Korea has built, mostly at its own cost, the massive US bastion in Pyeongtaek, south of Seoul – America’s largest overseas base.

Alliance management is no easy task. South Korea’s peculiarities, perhaps, make it a trickier ally than most. But its strategic value remains.

Anti-Americanism + anti-Japaneseism = trouble

If the US top brass strong-arm Seoul into emptying its coffers to cover joint defense costs, while also demanding a humiliating about-face on GSOMIA, they run significant risks.

Currently, South Korea is one of the few nations on Earth where both the right – who admire his conservative stance – and the left – who admire his engagement with North Korean leader Kim Jong Un – respect US President Donald Trump. Meanwhile, nationalist furies are deployed in full cry against Japan.

But if currently quiescent anti-Americanism – which exploded in 2002, after the gruesome death of two schoolgirls in a road accident with US troops – reignites alongside anti-Japaneseism, Washington is in uncharted waters. That so-far-unrealized double whammy could seriously endanger long-term US strategic interests in Northeast Asia.

Korean conservatives already fear that South Korea could be leveraged away from maritime, democratic Northeast Asia – Japan and Taiwan – toward continental, authoritarian powers – China, North Korea and Russia.

That fear is bolstered by two factors. Economically, China has long replaced the United States as South Korea’s main trade partner. Strategically, there are worries that an isolationist America could turn its back on the region, leaving the field to a rising China.

So, just as the US warns Seoul – in thinly disguised diplomatic code – that its constant prods against Tokyo play into the hands of Beijing and Pyongyang, an undiplomatic US approach toward South Korea could achieve the same end.

Hence the need, over the coming days, for Milley and Esper to wind their necks while dealing with their South Korean counterparts.

Could a face saving solutions be found? Could a compromise be reached on cost-sharing?

And could the United States, perhaps, upgrade its existing (but clunky) trilateral liaison function to enable better military information-sharing among Seoul, Tokyo and Washington, thereby obviating GSOMIA?

Supporters of a long and strong US presence in the region may well hope so.

Moreover, once they return to US soil, the general and the defense secretary have a related mission. That is, to sell the strategic value of forward-deployed troops in the region to two uninformed and/or skeptical audiences: the American people and their own boss – the isolationist American president himself.