China Primer: South China Sea Disputes

China Primer: South China Sea Disputes

China Primer: South China Sea Disputes

Updated December 22, 2025 (IF10607)

Overview

Multiple Asian governments assert sovereignty over rocks, reefs, and other geographic features in the heavily trafficked South China Sea (SCS), with the People's Republic of China (PRC or China) arguably making the most assertive claims. The United States makes no territorial claim in the SCS and takes no position on sovereignty over any of the geographic features in the SCS, but U.S. officials have long urged that disputes be settled without coercion and on the basis of international law. Separate from the sovereignty disputes, the governments of the United States, China, and other countries disagree over what rights international law grants foreign militaries to fly, sail, and operate in a country's territorial sea or exclusive economic zone (EEZ). Since the 113th Congress, some Members have examined China's efforts to use coercion and intimidation to increase its influence in the SCS and have passed legislation aimed at improving the ability of the United States and its partners to protect their interests.

An estimated one-fifth to one-third of global ship-borne commerce transits the SCS each year, including energy supplies to U.S. allies and partners like Japan, South Korea, and Taiwan. According to estimates cited by the U.S. Energy Information Administration, the SCS contains "proved" or "probable" reserves of approximately 3.6 billion barrels of oil and 40.3 trillion cubic feet of natural gas. The SCS also contains significant fish stocks, coral, and other undersea resources.

Disputes over Sovereignty

PRC officials assert "indisputable sovereignty over these islands [of the SCS] and their adjacent waters" without defining "adjacent waters." The PRC government depicts its claims with a "dashed line" (see Figure 1) that encompasses approximately 62% of the SCS, according to the U.S. Department of State. The PRC has never explained definitively what the dashed line signifies.

In the northern part of the SCS, the PRC, Taiwan, and Vietnam contest sovereignty of the Paracel Islands; the PRC has occupied them since 1974. The PRC and Taiwan also claim Pratas Island, which Taiwan controls. In the southern part of the sea, the PRC, Taiwan, and Vietnam each claim all of the approximately 200 Spratly Islands, while Brunei, Malaysia, and the Philippines, a U.S. treaty ally, claim some of them. Vietnam occupies the most land features in the island chain; Taiwan occupies the largest. In the eastern part of the sea, the PRC, Taiwan, and the Philippines claim Scarborough Shoal; the PRC has controlled it since 2012. PRC and Taiwanese claims overlap with the theoretical 200-nautical-mile (nm) EEZs that five Southeast Asian countries—Brunei, Indonesia, Malaysia, the Philippines, and Vietnam—could claim from their mainland under the 1994 United Nations Convention on the Law of the Sea (UNCLOS).

Figure 1. The South China Sea

Source: CRS. Boundaries from U.S. Department of State.

Dispute over Freedom of the Seas

A dispute over how to interpret UNCLOS underlies U.S.-China tensions over U.S. military operations in and over the SCS and other waters off China's coast. The United States and most other countries interpret UNCLOS as giving coastal states the right to regulate economic activities within their EEZs, but not the right to regulate navigation and overflight through the EEZ, including by military ships and aircraft. China, Vietnam, and some other countries hold the minority view that UNCLOS allows them to regulate both economic activity and foreign militaries' navigation and overflight in their EEZs.

The U.S. Navy routinely operates in the SCS and the Taiwan Strait, including transits of the Taiwan Strait and Freedom of Navigation Operations (FONOPs) near Spratly and Paracel Islands to challenge maritime claims that the United States considers to be excessive. U.S. Air Force and Navy aircraft fly surveillance and reconnaissance missions in international airspace above the waters of the SCS, including airspace that is close to (but outside of) China's airspace. China regularly conducts military activities in the SCS and objects to U.S. military activities there. PRC officials often say that U.S. military operations in the SCS undermine regional stability. Other countries—including Japan, Australia, Canada, the United Kingdom, and India—also have operated naval vessels and aircraft in the SCS to promote a free and open Indo-Pacific.

The PRC and the other SCS claimants (except Taiwan, which is not a member of the UN) are parties to UNCLOS. The United States is not a party, but has long had a policy of abiding by UNCLOS provisions relating to territorial waters, EEZs, and navigational rights. UNCLOS allows state parties to claim 12-nm territorial seas and 200-nm EEZs around their coastlines and "naturally formed" land features that can "sustain human habitation." Naturally formed land features that remain above water at high tide, but which are not habitable, are entitled to 12-nm territorial seas, but they are not entitled to 200-nm EEZs.

Flashpoints

Dangerous Encounters

The U.S. and other governments have accused PRC military and non-military ships and aircraft of conducting unsafe maneuvers in and over the SCS that put other regional actors at risk. The 2024 U.S. Department of Defense (DOD; also known as the Department of War) report on China's military notes that in 2023 "the PRC frequently adopted unsafe and unprofessional measures," though the report notes a reduction in the number of "coercive and risky air intercepts of U.S. platforms compared to the previous two years." U.S. officials argue that some of these behaviors were "inconsistent with" bilateral and multilateral agreements regarding air and maritime safety to which China is party.

Tensions Between China and the Philippines

For several decades, tensions in the South China Sea have periodically flared between China and other claimants (the PRC and Vietnamese navies engaged in armed combat over some of the islands in the 1970s and 1980s, for example). For more than a decade, tensions have been greatest between China and the Philippines, which refers to the area of the SCS within its EEZ as the West Philippine Sea. In 2013—the year after China gained de facto control of Scarborough Shoal following a confrontation between PRC and Philippine ships—the Philippines sought arbitration under UNCLOS over PRC actions in the SCS. In 2016, an UNCLOS arbitral tribunal ruled (among other things) that China's dashed line claim had "no legal basis," none of the land features in the Spratly Islands are capable of generating an EEZ, and China violated the Philippines' sovereign rights by interfering with Philippine vessels, damaging the maritime environment, and engaging in reclamation work on a feature in the Philippines' EEZ. The United States has urged China and the Philippines to abide by the ruling, which under UNCLOS is binding on both parties. China declared the ruling "null and void."

The China Coast Guard and PRC maritime militia regularly harass and block Philippine coast guard and fishing vessels throughout the SCS, including at Scarborough Shoal and Second Thomas Shoal, which houses a marine detachment on a grounded Philippine Navy vessel, the BRP Sierra Madre. After a series of incidents in which PRC vessels prevented the resupply of the Philippine ship, in July 2024 the Philippines and China agreed to allow Philippine vessels to deliver supplies to the marine detachment. In September 2025, China declared a nature reserve at Scarborough Shoal; both the United States and the Philippines have rejected the declaration as unlawful.

Renewed tensions could have implications for the United States. In 2023, the United States and the Philippines announced Bilateral Defense Guidelines to strengthen their Mutual Defense Treaty, stating that armed third-party attacks against Philippine armed forces, including Coast Guard, aircraft, or public vessels "anywhere" in the SCS, would invoke U.S. mutual security commitments under the treaty. Trump Administration officials have reaffirmed the "ironclad" U.S. commitment to the Philippines.

China's Artificial Islands

Between 2013 and 2015, China undertook extensive land reclamation (i.e., island-building operations) in the Spratly Islands. According to DOD estimates, that reclamation created around five square miles of artificial landmasses on the seven disputed sites that China controls. China built military infrastructure on and deployed advanced anti-ship and anti-aircraft missile systems and other military equipment to the outposts. The 2024 DOD report on China's military stated that these outposts "enable the PRC to maintain a more flexible and persistent military and paramilitary presence in the area," which "improves the PRC's ability to detect and challenge activities by rival claimants or third parties and widens the range of response options available to Beijing." Since 2021, Vietnam has accelerated reclamation and construction work on SCS features it controls.

Regional Cooperation

China and the Association of Southeast Asian Nations (ASEAN) have been negotiating a code of conduct for parties in the SCS since 2002. Observers say that a binding code is unlikely and allege that China has prolonged the negotiations to buy time to carry out actions aimed at further strengthening its position in the SCS.

The U.S. government has sought to enhance allies' and partners' maritime domain awareness in the SCS. In 2022, the Quadrilateral Security Dialogue—a grouping of the United States, Japan, Australia, and India—announced an effort to improve maritime domain awareness in the Indo-Pacific, including the SCS. The U.S. Navy conducts patrols and exercises in the SCS with regional partners, including Australia, Canada, India, Japan, and the Philippines.

Select Legislation

Under a security assistance program currently known as the Indo-Pacific Maritime Security Initiative, authorized by Congress in the National Defense Authorization Act (NDAA) for Fiscal Year (FY) 2016 (P.L. 114-92) and modified in subsequent NDAAs, the United States has sought to improve the ability of regional countries to enhance maritime domain awareness and patrol their EEZs. The NDAA for FY2021 (P.L. 116-283) established a Pacific Deterrence Initiative (PDI) to strengthen U.S. defense posture in the Indo-Pacific region, addressing issues such as those in the SCS. Congress extended and expanded the PDI in subsequent NDAAs, including the FY2026 NDAA (P.L. 119-60).