Whenever they meet, officials from South Korea and the United States have a favoured phrase for describing the strength of their relations.

The three-word motto “We go together” is intended to encapsulate the common cause that has united the sides in a military alliance for more than six decades. When President Donald Trump was welcomed to South Korea on a rare state visit in November 2017, he was gifted traditional Korean utensils engraved with the phrase.

But behind the rhetoric, cracks in that unity have raised questions about the future of the alliance, a bulwark against nuclear-armed North Korea and a major plank of a US security policy focused on regional stability and Chinese containment.

The US-Korea alliance is experiencing its greatest crisis in recent years
Song Dae-sung

In Washington and Seoul, observers fear a perfect storm of risks to a partnership that is widely seen in both capitals as having been a linchpin of peace and prosperity since the end of hostilities between the US-backed South and communist-backed North in 1953.

Those headwinds include a US president with isolationist tendencies, a dovish South Korean administration accused by critics of putting rapprochement with Pyongyang ahead of denuclearisation, and left-wing-nationalist opposition in South Korea to the US military presence there.

Meanwhile, North Korea, whose leader Kim Jong-un held a surprise meeting this week with Chinese President Xi Jinping – possibly to strategise ahead of his expected second summit with Trump and a visit to Seoul – stands ready to exploit any opportunity to divide the allies.

“I think the US-Korea alliance is experiencing its greatest crisis in recent years,” said Song Dae-sung, former president of the Sejong Institute, a security think tank in Seoul.

For the allies, 2019 could hardly have begun on a more ominous footing.

After a year of failed negotiations, the deadline for Seoul and Washington to renew an agreement on sharing the cost of hosting 28,500 US troops in South Korea expired on December 31.

In South Korea, resentment of refugees from the North

With talks spilling into the new year, the sides have struggled to reach an agreement as the Trump administration pushes for a massive increase in South Korea’s contribution, which has been renegotiated every five years since 1991.

Seoul currently pays 960 billion won (US$860 million) towards the upkeep of US Forces Korea, almost half of the total.

While South Korea bumped up its share by 6 per cent after the last round of successful negotiations in 2013, the US president is reported to be seeking up to 150-200 per cent of the current amount.

Since the end of hostilities in the Korean war, successive leaders in the US and South Korea have praised the alliance for preventing conflict with North Korea and ensuring regional stability.

In a sign of their long-term commitment, the two sides in 2004 agreed to a massive expansion of Camp Humphreys, about 65km south of Seoul, to create the biggest US military base located overseas.

Trump, however, has repeatedly complained about the cost of supporting the defence of allied countries including South Korea, and hinted at the possibility of drawing down or withdrawing US troops.

“He questions whether we need that big a military presence in South Korea,” said a former Trump administration official, speaking on condition of anonymity.

“It’s part of the ‘America First’ platform, that we need to bring as many of these men and women back home as soon as possible. We shouldn’t be out there all over the world.”

On Tuesday, Trump returned to the theme on Twitter by railing against “rich countries that so greatly benefit from what we are doing” while providing “little financial or military help”.

The US Department of Defence and United States Force Korea did not respond to requests for comment.

“Most Koreans strongly believe that South Korea pays a large portion of the cost associated with keeping US troops in this country,” said Kim Jong-ha, dean of the Graduate School of National Defence and Strategy at Hannam University in Daejeon, South Korea.

The other time Singapore gave North Korea a taste of America

“They have a reasonable suspicion that President Trump, as a businessman, is not personally committed to the US-South Korea alliance. The main worry of many Koreans is that the US could leave the South exposed by either withdrawing from the peninsula or sitting out any North-South conflict for fear of North Korean nuclear strikes against the US.”

Although South Koreans overwhelmingly support the alliance – 96 per cent viewed it as “necessary” in 2017, according to an opinion poll by the Asan Institute for Policy Studies – many have grown frustrated with Trump’s “America First” posturing.

In an editorial last month, the reliably pro-American, right-leaning Joongang newspaper warned that Washington’s “excessive demands” related to cost sharing risked sparking an anti-US backlash in the country.

“South Koreans are really puzzled why President Trump perceives the alliance only in terms of money,” said Choi Kang, vice-president of the Asan Institute of Policy Studies in Seoul. “The alliance is more than money.”

“We were able to solve the differences between the US and Korea over the past 26 years, over this issue,” said Choi. “But this time, actually, officials are frustrated.”

Anxiety about the US commitment has been exacerbated by the resignation last month of Defence Secretary James Mattis, the so-called “last adult in the room” who staunchly defended Washington’s partnerships abroad.

“He represented a symbol, of defender of the alliance. But [now] who is going to step up to say we should preserve the alliance we have cherished since the end of World War Two?” said Choi.

“There is no one who can say ‘no’ to President Trump on the US side.”

Uk Yang, a defence expert at Shin Ansan University, said there were “so many unknowns”.

“We don’t know what North Korea will do with denuclearisation, since it’s going nowhere right now. So without results, Trump will change his view or position for North Korea and then all hell will break loose.”

FRUSTRATION BREWS

In October, South Korean Foreign Minister Kang Kyung-wha offered a rare acknowledgement of frustration brewing in Washington over the differences between the sides on how to kick-start stalled progress on North Korean denuclearisation.

In a phone call, US Secretary of State Mike Pompeo had expressed “discontent” over an inter-Korean military pact in which the Koreas agreed to cease military drills and remove guard posts and landmines along their heavily militarised border.

The deal was seen as giving major concessions to the North, which made a vague commitment to the “complete denuclearisation of the Korean peninsula” during the Trump-Kim summit in Singapore in June, before it had made tangible steps towards disarmament.

Kang’s admission made explicit tensions stemming from a perception in Washington – and among conservatives and security hawks in South Korea – that left-leaning South Korean President Moon Jae-in is far more concerned with reconciliation with the North than its nuclear arsenal.

“There is daylight, obviously, in the policy positions between Washington and Seoul,” said Soo Kim, a former North Korea analyst with the CIA. “We are still maintaining the line where we are not going to ease up on sanctions unless North Korea ... would show some sort of credible, measured and irreversible steps toward dismantling some of their nuclear facilities.”

In a New Year’s press conference on Thursday, Moon welcomed a call by Kim Jong-un for the reopening of an inter-Korean industrial park and tours to a scenic mountain resort in the North – moves that would require the easing of US and UN sanctions.

“I think North Korea knows that they clearly have to denuclearise for the easing of international sanctions, and the US also understands that there needs to be corresponding action to expedite the North’s denuclearisation,” he said.

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Although Moon, a former special forces soldier, has repeatedly expressed support for the alliance – even suggesting it would survive Korean reunification – his administration has been dogged by controversy over the views of some of its senior figures.

Im Jong-seok, until recently Moon’s chief of staff, was jailed during the 1980s for helping to organise an illegal trip by a student activist to North Korea to meet its late founder Kim Il-sung – although associates have insisted his views have since moderated.

In May, the administration was forced to clarify that presidential adviser Moon Chung-in spoke for himself when he argued in Foreign Affairs that signing a peace treaty with the North would make it “difficult to justify” US forces staying in South Korea.

South Korea’s presidential Blue House did not respond to a request for comment. But in an apparent effort to assuage concerns, the South Korean president told reporters on Thursday that the alliance was separate from denuclearisation talks and solely a matter for Washington and Seoul.

“The US still holds doubts about the possibility of South Korea pursuing its own North Korean policy without prior coordination with the US,” said Kim of Hannam University. “In short, the key, or core of the alliance is credibility. However, the current state of the US-South Korea alliance has a problem with credibility.”

One man likely to be happy with the simmering tensions between the allies is the North’s Kim, who completed his fourth trip to Beijing on Wednesday ahead of as-yet unscheduled summits with Trump and Moon.

Pyongyang has long demanded the removal of “imperialist aggressor forces” in South Korea, viewing them as an obstacle to the unification of the Korean peninsula. China, too, views the alliance with suspicion, perceiving it as a challenge to its strategic dominance and fearing the prospect of US troops on its border in the event of Korean unification.

China’s state news agency Xinhua reported on Thursday that Xi had told Kim of his support for a second summit with Trump and hoped the pair would “meet each other halfway”.

In his New Year’s address, Kim did little to hide his true feelings about the close relationship between the US and the South. North and South should “join hands”, he declared, while “interference and intervention of outside forces who stand in the way of national reconciliation, unity and reunification” would not be tolerated.

“He clearly stated that if South Korea wants more cooperation with North Korea, you have to make a choice,” said Choi of the Asan Institute. “You have to be more independent from the United States.”

Analysts say that Kim sees his meetings with Trump and Moon as a chance to exploit existing tensions to drive a wedge between Washington and Seoul.

“He’s dealing with a South Korean administration that has been very supportive, in fact sometimes even facilitating his goals,” said Kim, the former CIA analyst. “Looking at the political situation here in the United States, and the political climate in South Korea, the stars, I would say, are pretty well aligned for Kim.”