IN SOUTH KOREA economic forces and security policy seem to be in a tug-of-war: commercial ties with China are ever more important, yet the country relies for its defence on an alliance with the United States. So far, so like Japan. But unlike Japan, South Korea enjoys excellent relations with China, while it faces a serious military threat from a third party, its bellicose and now nuclear-armed neighbour, North Korea. The tension in South Korea between reliance on China’s economy and America’s armed forces can be acute. As the Korea Times, an English-language newspaper, put it this week: “South Korea is walking a tightrope”.

This week, too, Japan’s prime minister, Shinzo Abe, visited Washington, DC, agreeing with Barack Obama’s administration on a revision of guidelines covering the two countries’ defence co-operation, in effect strengthening the bilateral alliance. That is reassuring to Japan, given the risk that its territorial dispute with China might lead to a confrontation. But for South Korea, managing the alliance with America, which has nearly 30,000 troops on its soil, is more delicate. Both America and China harbour suspicions about the other’s intentions. South Korea, close to both, at times finds itself at odds with one or the other.

This year South Korea has already defied America as one of several allies that ignored advice to shun the new Asian Infrastructure Investment Bank (AIIB) that China is setting up. But South Korea has also issued a rare rebuke to China. In March, after what a Chinese deputy foreign minister visiting South Korea called “a very candid and free dialogue” (ie, a blazing row), a spokesman for the South Korean defence ministry sniffily noted that although a “neighbouring country” might have its own opinions, “it should not try to influence our security policy.”

At issue was America’s suggestion that it might deploy a new missile-defence system in South Korea, known as Terminal High Altitude Area Defence, or THAAD. America says this is needed to protect South Korea from the threat of conventional or nuclear missile attacks from the North (presumably America hopes it would help defend Japan as well). China, however, claims that the deployment is directed against it. In October it said THAAD would damage “peace and stability in the region” and it warned “relevant countries” that they should “not take their own security concerns as excuses for damaging others’ security interests.”

South Korea has plenty of reasons not to anger China, which takes a quarter of its exports and is also its biggest source of imports. (In February China and South Korea reached agreement on a free-trade deal.) Helped by the apparent personal rapport between the two presidents, Park Geun-hye and Xi Jinping, relations between South Korea and China have blossomed. Both sides are united in their suspicion of Mr Abe, whom they accuse of trying to rewrite the history of Japanese militarism in the first half of the 20th century—and even of wishing to revive it. Moreover, China’s traditional ties with North Korea increase its importance as a partner for the South. No longer are China and North Korea “as close as lips and teeth”, and China has yet to receive the young dictator, Kim Jong Un, for a visit. But it remains the only country capable, in theory, of exerting any restraining influence on his erratic regime.

Fantasy reunification: What the two Koreas would gain from each other if Kim Jong Un's regime fell Not that North Korea shows much evidence of restraint. Last year it conducted 19 missile tests involving at least 111 projectiles. Test firings into the sea have continued this year, including of two missiles in early April, just before America’s defence secretary, Ashton Carter, arrived in Seoul, the South’s capital. A senior American military officer has postulated that North Korea may now have the capacity to launch intercontinental missiles from a mobile launcher. In February in Beijing, a Chinese expert told a seminar with some American counterparts that North Korea may already have as many as 20 nuclear warheads, twice as many as had been thought. At a conference in Seoul this week organised by the Asan Institute, a think-tank, a former Chinese diplomat once deeply involved in nuclear negotiations with North Korea, Yang Xiyu, said that even if ten was the more likely number, a new North Korean nuclear test (the fourth) was a matter of “not whether, but when”. The debate in South Korea about how the country should defend itself is not resolved, however. Both South Korean and American officials say formal talks on THAAD have not yet begun. Deployment, if it happens, is at least two years away. It will be controversial, needing a lot of land, and an exclusion zone because of the dangers to health and the environment of the radar that the system uses. Some in South Korea advocate another system, developed indigenously. Nor is it clear who would pay for the expensive installations.

THAAD’s not the way to do it

So it is puzzling that China should have made such a public fuss about THAAD now. Experts dismiss its ostensible worries about its purpose: the intrusiveness of the radar, and the “interoperability” of the system—ie, the fear that it might be linked up with those already deployed in Japan and Guam, and that it might eventually even be rolled out in India, encircling China. In fact, say analysts, China knows THAAD will have no impact on the effectiveness of its missiles. Nor is it likely to provoke more than the usual hysterical reaction from North Korea.

Some suspect that it is for symbolic reasons that China has decided to pick a fight—to show that in the emerging Asia which China expects to dominate, it wants a say in decisions that it reckons affect its own security. If so, it seems to have overplayed its hand, antagonising South Korea by bullying it—just as America blundered when it tried to stop its allies joining the AIIB. The moral seems to be that China’s economic clout can win it commercial acquiescence, but that when it comes to arm-wrestling over matters of national security, America still has the muscle.