Relations between China and South Korea have been quickly descending over the past year.

J Berkshire Miller is the director of the Council on International Policy.

Earlier this month, two Chinese fishing vessels rammed and sunk a South Korean Coast Guard ship in response to Seoul’s attempt to enforce against illegal fishing off the west coast of the Korean peninsula.

The lost Korean vessel was trying to enforce laws against a quasi-fishing militia of more than 40 Chinese ships. Fortunately, both sides avoided a more harmful clash when reinforcements from the Korean Coast Guard defused the situation and scattered the remaining Chinese vessels after firing in the direction of the boats.

South Korea has responded to the aggressive move by summoning the Chinese ambassador in Seoul and promising that any future incidents of illegal fishing would be met by armed force.

Meanwhile Beijing has tried to downplay the situation and has urged Seoul, unconvincingly, not to overreact.

Tense relationship

This incident, which fortunately did not result in the deaths of any Coast Guard officials, sheds light on the increasingly tense relationship between China and South Korea.

Indeed, relations between China and South Korea have been quickly worsening over the past year, following an initial honeymoon period under the leadership of Xi Jinping and Park Geun-hye respectively.

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Initial cracks in the bilateral relationship started to form with China’s decision to unilaterally declare an Air Defense Identification Zone (ADIZ) in the East China Sea in the fall of 2013.

The idea that Beijing would noticeably change policy on North Korea soon became a fantasy, despite China's almost unavoidable acceptance of new United Nations Security Council sanctions on Pyongyang earlier this year.

While the move was primarily intended to turn the screws on Japan, with which Beijing has had a long-standing territorial row over the Senkaku islands, China made an inexplicable error in extending the ADIZ to include Ieodo Reef – a somewhat latent maritime feature in dispute which is controlled by Seoul but claimed by Beijing.

The overlap of the ADIZ and – more importantly – the failure to communicate the decision well in advance to Seoul irritated South Korea and caused a hiccup in the positive trajectory of relations between Beijing and Seoul.

The second significant disappointment for Seoul was the lack of return on its political investment with Beijing – especially in terms of China’s relationship with North Korea. Park seemingly was prepared to open a new era of relations with Beijing, even at the cost of its reputation in the United States and Japan, if China could produce tangible efforts to rein in the regime of Kim Jong-un in North Korea.

The idea that Beijing would noticeably change policy on North Korea soon became a fantasy, despite China’s almost unavoidable acceptance of new United Nations Security Council sanctions on Pyongyang earlier this year.

A tougher approach

Since the beginning of the year, the North has conducted two nuclear weapons tests and a barrage of ballistic missile tests aimed at refining its range and accuracy.

North Korea has conducted two nuclear weapons tests since the beginning of the year [Reuters]

Meanwhile, Pyongyang continues to stockpile fissile material for its nuclear weapons programme and has also been working on perfecting the process of miniaturisation of a nuclear warhead to be fitted on top of one of its missiles.

Beijing has incrementally taken a tougher approach to the North, but it remains Pyongyang’s only real economic lifeline and cross-border trade between the two has been growing rather than subsiding.

Simply put, China’s urge for calm and the avoidance of “provocative actions” – such as its veiled critiques of joint US-Korea military exercises – fall short of any diplomatic consensus that Seoul had in mind.

Indeed, Park even went as far as attending Beijing’s ostentatious military parade in 2015 to celebrate the 70th anniversary of the end of World War II. Park’s dubious attendance at the ceremony, which raised eyebrows in Washington and elsewhere, was ostensibly an extension of goodwill to Beijing with the expectation that Xi would take a more favourable position on North Korea.

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North Korea’s sustained provocations – and its lack of interest in denuclearisation – have forced Seoul’s hand to lean more heavily towards deterrence. This has pushed South Korea even closer towards its alliance with Washington and also helped to partially reverse the negative trajectory of relations with Japan.

Chinese interests

Both of these developments are antithetical to Chinese interests but ironically have been facilitated largely by Beijing’s own policies vis a vis the North. Indeed, Seoul decided to go ahead with its decision to deploy the US Terminal High Altitude Area Defense (THAAD) advanced anti-missile shield as a response to Pyongyang’s sustained provocations.

The decision to deploy THAAD has enraged Beijing, which – irrationally – believes its strategic interests are directly threatened by the missile defence battery. South Korea has also drawn closer to Japan on security issues and signed a trilateral military information-sharing agreement – with their mutual US ally – in late 2015.

Over the past three years, Park and Xi have held numerous summits and at times have gushed at the possibilities of the bilateral relationship. The two sides also signed a free trade agreement and remain deeply connected economically. Despite this, the honeymoon period of relations between Seoul and Beijing has subsided and hard geostrategic divergences have come to fore.

J Berkshire Miller is the director of the Council on International Policy and is a fellow on East Asia for the EastWest Institute.

The views expressed in this article are the author’s own and do not necessarily reflect Al Jazeera’s editorial policy.