About one-third of the world’s trade passes through the South China sea, according to the U.N., including crucial energy supplies for U.S. allies Japan and South Korea. The sea also includes oil and natural gas reserves as well as valuable fishing grounds, coral and minerals.

China claims much of the South China Sea. In its southern part, those claims conflict with those of the Philippines, Vietnam, Indonesia, Malaysia and Brunei and in the northern part with those of Taiwan. And the claims of some of these Southeast Asian countries also overlap.

The U.S. has not endorsed any of these claims, urging that disputes be settled peacefully and on the basis of international law. The U.S. insists on freedom of navigation through these contested waters and has repeatedly sailed warships through them to assert that right.

The U.S. has also committed under a 1951 mutual defense treaty with the Philippines to respond if its armed forces, vessels or aircraft are attacked.