Seoul maintains the system is a deterrent in the face of threats from North Korea, with analysts differing on its effectiveness. Beijing says the THAAD radar could be used to spy on its military activities and poses a security threat.

Relations between Beijing and Seoul – already playing out amid heightened sensitivity to Washington’s moves in the region – will be further strained by the development, according to Koh King Kee, director of the Asean Research Centre, at the Institute for Community with Shared Future, a Communication University of China think tank.

“THAAD expansion will unquestionably affect the stability of the Asia-Pacific region. China will not take it lightly, especially after the recent cross-strait crisis created by US House Speaker Nancy Pelosi’s visit to Taiwan,” he said.

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US House Speaker Nancy Pelosi arrives in Taiwan as Beijing announces live-fire military drills

US House Speaker Nancy Pelosi arrives in Taiwan as Beijing announces live-fire military drills

Koh pointed out that Russia, which has accused the battery of being a “dual use” system that could fire “attack missiles”, has similarly opposed the project. He also suggested the latest development could trigger a more aggressive arms race between Seoul and Pyongyang.

But he said Beijing is unlikely to immediately impose economic sanctions on Seoul, although South Korea’s economic dependence on the Chinese market makes it easy to do so.

“In 2020, close to 30 per cent of South Korea’s import of material parts came from China, particularly semiconductors, large-capacity batteries, rare earth metals and medical supplies. Korean economies could be materially hit should China restrict or reduce export of such crucial material parts.”

Trade and cultural exchanges – halted by Beijing for a few years – only resumed when former president Moon Jae-in committed to a “three noes” policy in 2017 – no additional deployment of THAAD batteries, no South Korean integration into a US-led regional missile defence system, and no trilateral alliance with the US and Japan.

With an eye on China, South Korea looks to build ‘supply chain alliances’

But Moon’s successor Yoon, who observers have described as “pro-West”, said he would not abide by the “three noes”, and that the policy was not a commitment to China or a formal agreement with China, but a statement of Moon’s own political position.

According to A.B. Abrams, an East Asian security expert and author of Immovable Object: North Korea’s 70 Year War with American Power, THAAD’s viability to protect targets in South Korea against northern missile strikes is “questionable”.

He believes the lower-altitude and short-range missiles used on the southern peninsula “will fall well outside its optimal targeting altitudes”.

Abrams said the system would be of greater value to the US amid the Pentagon’s “growing focus on confronting China since the early 2010s”.

“Its sensors provide an early warning of ballistic missile launches from China, Russia and North Korea, which could provide cueing information to missile defences at sea and on Guam, Hawaii and the US mainland,” he said.

China’s top legislator urges cooperation with South Korea on supply chains

Abrams, who has lived and researched in both parts of the peninsula, said there is opposition within South Korea to THAAD because it makes the country a “priority target” in any war between the US and China or Russia.

“Other reasons for opposing deployments have included reports of possible adverse effects on locals’ health, the detrimental effects they have had for relations with South Korea’s largest trading partner China, and the fact that they continued in 2017 in secret, despite the newly inaugurated Moon Jae-in administration barring them at the time,” he said.

“A missile defence system controlled by Koreans, rather than Americans, is expected to be far less contentious for Beijing and potentially present a path to easing tensions.”