Last week, Japan became the latest country to submit a diplomatic note to the United Nations on the South China Sea , rejecting China’s position that its drawing of territorial sea baselines around the islands and reefs it claims in the waters conforms to the UN Convention for the Law of the Sea and international law. A tribunal in The Hague had in 2016 ruled in a dispute brought forth by the Philippines against China that some maritime features in the South China Sea were “low tide elevations” that did not generate sovereignty claims nor enjoy maritime entitlements. Still, Tokyo in its note pointed out that China had asserted its “sovereignty” in sea and airspace surrounding and above those maritime features found to be low-tide elevations, and protested “overflight of Japanese aircraft in the airspace surrounding Mischief Reef and attempted to restrict the freedom of overflight in the South China Sea”. The timing of Japan’s note, on the eve of a new president taking office in the United States – its long-time military ally – is significant. The administration of President Joe Biden is expected to seek a more multilateralist and “rules-based” approach in its policies toward China, including on the South China Sea issue. Japan’s note verbale is perhaps its way of reinforcing its support for its ally, a gift of sorts to the new leadership. Japan has in recent years paid closer attention to the South China Sea , strengthening its military position there. Last October, it dispatched the helicopter carrier Kaga DDH-184, the destroyer Ikazuchi DD-107, and the submarine Shoryu SS-510 for a joint anti-submarine exercise in the contested waterway. Under former Prime Minister Shinzo Abe , Japan pushed for its military to play a more active role to target enemies on land, rather than stopping attackers in the air and sea. Aiming to have more “normal” military power and grow its influence in the Asia-Pacific region, Japan is likely to respond positively to any US request for joint military operations such as freedom of navigation operations in the South China Sea. Its diplomatic note may also be intended as a method of constraining China when it comes to the territorial and maritime disputes that the two have in the East China Sea . Japan’s note comes more than a year after Malaysia in December 2019 made an application to the Commission on the Limits of the Continental Shelf to establish an extended continental shelf for the country beyond the usual 200 nautical miles, in the northern part of the South China Sea. Since then, other countries in the region have also submitted note verbales – given to the UN Secretary general with a request that they be circulated to other member states – challenging China’s position, with the United States, Britain, Germany, France and Australia following suit. Beijing believes the 2016 tribunal award is invalid and that its establishment of straight baselines around offshore islands – and assertion that the spaces within the baselines are China’s internal waters – do not violate international law. Marine features such as Mischief Reef are also part of the Spratly Islands, over which China has always claimed territorial sovereignty as one integral unit. It is worth noting that Beijing announced the straight baselines of the Paracel Islands in 1996, and the straight baselines of the Diaoyu Islands – also known as the Senkaku Islands in Japan – in 2012. In the meantime, despite the existence of territorial sovereignty and maritime delimitation disputes among claimant countries, freedom of commercial navigation and overflight in the South and East China Seas has not been disrupted. Japan’s note thus can be seen as another chapter of lawfare aimed at rekindling the issue, given that the last note was issued by the Philippines three months ago. One point worth highlighting, though, is that in its note verbale, Tokyo did not comment on the validity of Beijing’s territorial claims in the South China Sea, which other countries touched upon in their note verbales and which was the focus of the 2016 tribunal award. This is an outcome of delicate design. As a non-party in the South China Sea dispute, Japan wants to display its neutrality. Japan protests Chinese survey ship operating near Okinotorishima atoll At the same time, we might infer Japan’s self-interest at play here, given that it also has maritime claims that are being disputed. This relates to Okinotorishima – two uninhabited rocky outcroppings about 1,700 kilometres southwest of Tokyo. Japan says they are islands and claims the full exclusive economic zone, the continental shelf and even areas beyond the 200-nautical mile continental shelf for these features. But Beijing and Seoul say they are only two rocks remaining above the water surface at high tide, thus not qualifying as a point from which to demarcate Japan’s exclusive economic zone and continental shelf. The South China Sea dispute last year became a source of deepening tension between the US and China , while we saw a rise in lawfare from Southeast Asian states. Japan’s note at the start of the year shows that such legal challenges will continue and have an impact on efforts to stabilise the South China Sea situation in the coming years. Ding Duo is deputy director of the National Institute for South China Sea Studies’ Research Centre for Oceans Law and Policy in Hainan, China, and non-resident Research Fellow of the Institute for China-America Studies (ICAS) in Washington, DC.