Seoul - With a fundamental, strategic shift affecting the foundation of the South Korea-US alliance, and China competing strongly for South Korean kinship, Tuesday's state visit to Seoul will be a diplomatic minefield for US President Donald Trump.

Over two days, Trump will meet South Korean President Moon Jae-in and address the National Assembly. Foregoing a photo opp at the Korean Demilitarized Zone (DMZ) - a standard stop for visiting presidents - he will go to Camp Humphreys, a vast military base 50km south of Seoul that is rising as a new hub for the 28,500 US forces on the peninsula.

A critical undercurrent during the visit will be the ongoing shift in Washington's strategic focus, driven by North Korea's expanding nuclear capabilities.

Since the end of the 1950-53 Korean War, the defence of South Korea has been the Korea-US alliance's foremost priority. Now, with the United States increasingly in North Korea's crosshairs as it extends the range of its intercontinental ballistic missiles, Washington must consider its own defence.

This development is opening fissures in the alliance. While Trump has threatened to "totally destroy" North Korea, Moon told Korea's National Assembly last week that "our top priority is to maintain peace on the Korean Peninsula ... No military action on the Korean Peninsula shall be taken without prior consent of the Republic of Korea."

In fact, the Korea-US Mutual Defense Treaty mandates that both allies must consult prior to military action, but grants neither veto power.

"Every country has rights to self-defence," said Tara O, the Washington-based author of The Collapse of North Korea. "If the US is attacked by North Korea, the US can retaliate without any country's permission."

Yet Seoul's non-cooperation would render US operations - other than short, offshore attacks - unsustainable.

"If an actor is unable to sustain military operations on the peninsula, that military action would be futile in achieving any positive political objectives," Dan Pinkston, a Seoul-based international relations scholar at Troy University, told Al Jazeera.

Protests expected

Meanwhile, the allies are continuing long-term negotiations on the transfer of wartime operational control of Korean troops to a Korean, rather than US, general. The process has been rocky, particularly given recent North Korean hacks of South Korean defence networks.

While Seoul has announced plans for pre-emptive "Kill Chain" air/missile strikes against North Korea and "decapitation attacks" aimed at the country's leader, Kim Jong-un, experts say the strategy still relies heavily on US intelligence, reconnaissance and command assets.

Noisy demonstrations are expected during Trump's visit, with liberals deriding and conservatives supporting his stance on North Korea. Unlike his predecessors, Trump has threatened Pyongyang directly, eschewing diplomatic language and nicknaming its leader "Rocket Man". At the United Nations, he warned that the US was "ready, willing and able" to take military action against North Korea's strategic weapons if non-kinetic UN efforts failed.

This unfamiliar American bellicosity has alarmed many South Koreans. Famed Korean author Han Kang penned an October column in the New York Times confiding her deep fears about US war talk.

Seoul and the surrounding area, where half of South Korea's 50 million people reside, is vulnerable to retaliation from North Korean missiles, artillery and commandos, and military action could prompt a spiral of escalation leading to an atomic catastrophe.

"I think we need calm, strategic responses rather than provocations," Choi Jin-wook, a Korean scholar at Japan's Ritsumeikan University, told Al Jazeera, referring to Trump's rhetoric. "This is not a game."

China's role

Meanwhile, South Korea is walking a diplomatic tightrope between the US, its wartime saviour and strategic ally, and China, its wartime enemy and leading export and investment destination.

Last week, Seoul and Beijing agreed to mend ties frayed by the deployment of US THAAD (Terminal High Altitude Area Defense) systems to South Korea. Washington and Seoul contend that THAAD defends against North Korean missiles, while Beijing says THAAD's radars can snoop on its own systems.

Per the agreement, Beijing is expected to halt economic retaliation against Korean companies and permit resumption of various types of Chinese tours to South Korea, suspended amid the THAAD dispute. In return, Seoul has agreed not to deploy any additional THAAD batteries, nor to join the US missile defence shield.

Seoul also stated that current defence drills conducted by Japanese, South Korean and US forces would not morph into a trilateral alliance - a blow to Washington, which favours a regional democratic alliance.

"It was inevitable from President Moon's perspective, as Korea has been under pressure from THAAD, an American interceptor, but the US has not shared the pains suffered by Koreans," Oh Young-jin, editorial page editor of The Korea Times, told Al Jazeera. "This was a matter of necessity."

Other observers, however, were appalled. "It has settled the THAAD issue, but on the other hand, it sounds like China is dictating South Korea's foreign and national security policy," said James Kim, a scholar at Seoul's Asan Institute. "This is unprecedented - a huge development."

Bilateral trade

Further straining Korean-American amity is Trump's demand for revisions to a bilateral free trade agreement that took effect, following years of negotiations, in 2012.

As it grants Korean exporters tariff exemptions and subsequent price advantages over Japanese and Chinese competitors in the US market, Seoul only agreed to renegotiations next year with reluctance.

"Trump is shooting himself in the foot by pushing for this," Oh said. "He gave Koreans a sense of humiliation."

Furthermore, some Koreans fret that personal chemistry between Moon and Trump is lacking - unlike the good vibes Trump apparently shares with Japanese Prime Minister Shinzo Abe and Chinese President Xi Jinping. And since the departure of Barack Obama-appointee Mark Lippert in January 2017, the US ambassadorial post in Seoul has been vacant, leading Koreans to worry that they are being ignored.

"I am perplexed by the lack of appointments; failure to put talented folks in key positions cripples sound, informed policy-making," William Newcomb, a visiting scholar at the US-Korea Institute at Johns Hopkins, told Al Jazeera. "Officials in an acting capacity generally take care to restrict actions to simply managing affairs."