China’s sweeping claims of sovereignty over the sea—and the sea’s estimated 11 billion barrels of untapped oil and 190 trillion cubic feet of natural gas—have antagonized competing claimants Brunei, Indonesia, Malaysia, the Philippines, Taiwan, and Vietnam. As early as the 1970s, countries began to claim islands and various zones in the South China Sea, such as the Spratly Islands, which possess rich natural resources and fishing areas.

China maintains [PDF] that, under international law, foreign militaries are not able to conduct intelligence-gathering activities, such as reconnaissance flights, in its exclusive economic zone (EEZ). According to the United States, claimant countries, under UN Convention of the Law of the Sea (UNCLOS), should have freedom of navigation through EEZs in the sea and are not required to notify claimants of military activities. In July 2016, the Permanent Court of Arbitration at The Hague issued its ruling on a claim brought against China by the Philippines under UNCLOS, ruling in favor of the Philippines on almost every count. While China is a signatory to the treaty, which established the tribunal, it refuses to accept the court’s authority.

In recent years, satellite imagery has shown China’s increased efforts to reclaim land in the South China Sea by physically increasing the size of islands or creating new islands altogether. In addition to piling sand onto existing reefs, China has constructed ports, military installations, and airstrips—particularly in the Paracel and Spratly Islands, where it has twenty and seven outposts, respectively. China has militarized Woody Island by deploying fighter jets, cruise missiles, and a radar system.

To protect its political, security, and economic interests in the region, the United States has challenged China’s assertive territorial claims and land reclamation efforts by conducting FONOPs and bolstering support for Southeast Asian partners. Also in response to China’s assertive presence in the disputed territory, Japan has sold military ships and equipment to the Philippines and Vietnam in order to improve their maritime security capacity and to deter Chinese aggression.

The United States, which maintains important interests in ensuring freedom of navigation and securing sea lines of communication (SLOCs), has expressed support for an agreement on a binding code of conduct and other confidence-building measures. China’s claims threaten SLOCs, which are important maritime passages that facilitate trade and the movement of naval forces.

The United States has a role in preventing military escalation resulting from the territorial dispute. Washington’s defense treaty with Manila could draw the United States into a potential China-Philippines conflict over the substantial natural gas deposits or lucrative fishing grounds in disputed territory. The failure of Chinese and Southeast Asian leaders to resolve the disputes by diplomatic means could also undermine international laws governing maritime disputes and encourage destabilizing arms buildups.

Tensions between China and both the Philippines and Vietnam have recently cooled, even as China increased its military activity in the South China Sea by conducting a series of naval maneuvers and exercises in March and April 2018. Meanwhile, China continues to construct military and industrial outposts on artificial islands it has built in disputed waters.

The United States has also stepped up its military activity and naval presence in the region in recent years, including freedom of navigation operations (FONOPs) in January and March 2018. In a speech during his November 2017 visit to Southeast Asia, President Donald J. Trump emphasized the importance of such operations, and of ensuring free and open access to the South China Sea. Since May 2017, the United States has conducted six FONOPs in the region.