The Yellow Sea: An Overlooked Geopolitical Hotspot

South Korean intelligence revealed on 9 January that China had installed a steel structure in the Provisional Measures Zone (PMZ) of the Yellow Sea, an interim area where the exclusive economic zones (EEZs) of South Korea and China overlap. Although more details, including its location, were not disclosed, it is known to be a steel framework over 50 metres in both diameter and height, and China is reportedly planning to install a total of 12 such structures. This is not the first time China has constructed structures in the PMZ. Since 2022, China has been installing one or two structures annually, claiming them to be fishing facilities, yet some experts suspect that they could potentially be oil rigs. Regardless of their nature and purpose, these actions constitute a violation of the agreement between the two countries.

The Asiatic Mediterranean and the Yellow Sea

The Yellow Sea is a shallow body of water with an average depth of 44 metres, covering an area approximately 1.5 times the size of the UK and located between the Korean Peninsula and mainland China. Like many other littoral waters bordered by neighbouring countries, its width is limited, making it too narrow for South Korea and China to fully claim the 200 nautical miles of EEZ allowed under the UN Convention on the Law of the Sea, and the two countries agreed in 2015 to continue negotiating the delimitation. Despite the deterioration of relations starting in 2016 and the Covid-19 pandemic, negotiations have continued, yet little progress has been publicly reported. The most recent round of negotiations took place in Shanghai in November 2024. Given the timing of the meeting and the news of the revealed structure, one might question whether China is approaching these negotiations with sincerity. 

Although not enough is known to draw a clearer picture, this incident is a reminder of the geopolitical importance of the Yellow Sea, which well exceeds the limited attention it has been receiving. It is part of the Rimland, which, according to NJ Spykman, is critical for controlling Eurasia and the destiny of the world. The Bohai Sea, in particular, and more broadly the Yellow Sea, fall within China’s first island chain, connecting directly to the country’s centre of gravity. This area reflects China’s geopolitical vulnerability while serving as a conduit to the broader outer seas, including the East China Sea and beyond.

From China’s strategic point of view, solidifying its littoral defence and expanding the areas where it can enjoy freedom of action are critically important for its security. Gradualism is one of the key cultural notions applicable across China’s politics, economy and military. What we are witnessing is likely one of the steps China has consistently been taking, moving from near-coast defence toward having a true blue-water navy. While the image of China’s blue-water navy is often associated with the Indo-Pacific, or at least the South China Sea, from the perspective of Chinese gradualism, the foundations of a blue-water navy lie in establishing a robust littoral defence system.

The strategic value China places on the Yellow Sea is firmly grounded in its historical experiences, suggesting that this perspective is deeply entrenched and resistant to change. Most critical battles in Northeast Asia since the 19th century – a period China sees as a brief deviation from its rightful historical trajectory – have been fought in the vicinity of the Yellow Sea. The Second Opium War following the Arrow Incident (1856), the First Sino-Japanese War (1894–1895), and the Russo-Japanese War (1904–1905), as well as the subsequent settlements of these conflicts, all reflected the struggle for strategic dominance in the Yellow Sea. In a sense, it could be likened to the Turkish Straits between the Mediterranean and Black Seas, with access and control constituting a key aspect of strategic competition and significantly influencing the strategic balance, similar to how the Turkish Straits did in 19th-century European diplomacy. 

Strategic Implications

The geopolitical importance of the Yellow Sea is not limited to China; it also extends to South Korea and its allies and partners. For instance, the South Korean capital, Seoul, is connected to the Yellow Sea via the Han River, which flows through the heart of the city before reaching the sea. Two of South Korea's three naval commands are located along its shores. Additionally, USAG Camp Humphreys, one of the US’s largest overseas garrisons, is located within 10 miles of the shore and less than 400 miles across the sea from the People’s Liberation Army (PLA) North Sea Fleet headquarters in Qingdao.

Since 2013, China has been unilaterally insisting that the maritime operational demarcation with South Korea be set along the 124°E line. Although South Korea does not agree, Chinese military activities in the Yellow Sea, as well as the deployment of anti-ship cruise missiles and the installation of structures and buoys, continue to increase. These actions, of which the recent incident is an example, will continue as part of a broader strategy by China to secure its near seas and assert control over contested areas. 

Given the power disparity between the two countries, it is unlikely that South Korea alone could effectively counter China’s strategy of establishing a fait accompli in the Yellow Sea. In this regard, whether the most recent incident is related to South Korea’s domestic political situation or whether the construction of artificial structures will hold legal validity in any future demarcation between South Korea and China is beside the point.

What truly matters is that, unless this strategy is checked with firm resolve, the strategic balance in the region will ultimately shift in China’s favour, leaving the allies’ freedom of action restricted to a fraction of the Yellow Sea. This gradual shift would first weaken the defence posture for critical military assets along the west coast of the Korean Peninsula, gradually followed by Kyushu, Okinawa and ultimately Taiwan, triggering a domino effect and laying a solid foundation for further PLA expansion in the Western Pacific. In this respect, while the precise details of the incident remain unclear, it serves as a reminder of the enduring geopolitical importance of the Yellow Sea.

© RUSI, 2025

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