Illustration: John Shakespeare His actions speak louder than words. He has quickened the pace of testing. Under Kim Jong-un, North Korea is conducting underground nuclear explosions at an accelerated rate and ballistic missile tests much more frequently than at any time in the country's history. Together, these tests suggest a concerted effort to put miniaturised nuclear bombs on the tips of missiles capable of striking Japan, South Korea, and the giant US Pacific base in Guam, among other targets. Kim now has enough crude nuclear and missile capability "so that it is tantamount to an effective deterrent" against any attack, says an expert on international relations, Kim Tae-hyung of Soongsil University. "Could they really deliver nuclear warheads?" poses Professor Kim. "Maybe, maybe not, but we cannot assume they can't", and that gives Pyongyang a nuclear deterrent. The West's effort of the last 25 years was all designed to prevent this. It has failed.

Second, North Korea's hellbent efforts have had a profound effect in South Korea. For three years, South Korea had been intensifying a romance with its biggest trading partner, China. The US, Seoul's military ally, was much chagrined. South Korea's President Park Geun-hye had a remarkable six summits with her Chinese counterpart, Xi Jinping, in three years. Xi didn't hold a single meeting with his ally, North Korea's Kim Jong-un, in the same time. "The Korea-China relationship has become the best-ever national relationship in history," Xi declared last year, doubtless delighted to be bringing a US ally into China's sphere of influence. Strikingly, Park was the only leader of a US ally to go to Beijing for its big military parade last September. She sat in a place of honour on Tiananmen Gate with Xi, his wife and Russia's President Vladimir Putin. But when Park wanted to put her friendship with Xi to the test, she got a rude awakening.

North Korea conducted its fourth nuclear test in January. Park, alarmed, picked up the phone to speak to Xi. She, no doubt, wanted his help to rein in the rogue Kim. But Xi didn't answer her call. Or call her back. Park got the message. When it came to the crunch, the realpolitik of China's alliance with North Korea, however pathetic that country may be, trumped its relationship with South Korea. It was a threshold moment for Park. She took an equally realpolitik position. First, she withdrew all remaining co-operation with Pyongyang and struck a hard stance. "She is now aiming at a complete change of North Korean regime – not just withdrawing its weapons, but regime change," says Professor Kim. "I don't think that's compatible with China's policy", of supporting Kim's regime, preserving North Korea as a buffer state between itself and a US ally. He's right. It's not compatible. Even though China eventually supported new UN Security Council sanctions against North Korea, Park had made up her mind that she could not depend on Beijing in a crisis.

She turned to the US. In February, Park agreed to discuss installing a US missile shield system, the Terminal High Altitude Area Defence or THAAD, to defend South Korea against the missiles of the North. She has since committed South Korea to installing the system, effective in the second half of next year. China has reacted furiously. Its ambassador to Seoul said that the relationship between the two countries could be "destroyed in an instant" if it went ahead with THAAD. China's foreign affairs minister Wang Yi said that "we will not accept why they made a deployment exceeding the need". In other words, China demands to be the arbiter of South Korea's defence needs. In truth, China is worried that if South Korea has THAAD, it will make it immune not only to North Korean missiles but also to the coercive possibility of Chinese missiles. So when Kim fired his missiles last Wednesday, what did China do? Beijing not only failed to criticise his destabilising behaviour, it also intervened at an emergency meeting of the UN Security Council to prevent criticism of Kim for breaching a UN Security Council resolution. At the same time, Beijing last week started to punish South Korea for defying China.

In staccato, planned appearances in China by South Korean actors and singers were abruptly cancelled. The share price of South Korean entertainment firms fell by 5 to 25 per cent on Friday in response. At the same time, planned mass company trips to South Korea for Chinese workers were cancelled, aimed at damaging the South Korean tourist sector. These are undeclared Chinese economic sanctions against South Korea for acting in its own defence. Unfortunately, this is just the latest piece in the emerging picture of China as the great neighbourhood bully. Beijing is hitting out at countries that defy it – the running tally of countries subject to Chinese bullying now includes the Philippines, Vietnam and Indonesia in the South China Sea, Japan in the East China Sea and South Korea on the Asian landmass. North Korea is a deeply troubling rogue state, but we already knew that. The new and disturbing source of regional bullying is a much bigger and more serious power – China. "This shows China's true face," a South Korean official told me on condition of anonymity. "If it can happen once it can happen again – Koreans will now think twice about future investment in China." This is an emerging pattern that is troubling every capital across the Asia-Pacific and beyond.

Peter Hartcher is international editor. He travelled to Seoul as a guest of the South Korean government.