THE SOUTH CHINA sea stretches over 1.4 million square miles, rich in natural resources and bejeweled with islands. China has long regarded much of the sea as its own, claiming waters more than 1,000 miles from its shores and very close to the shores of other nations. Taiwan, Vietnam, the Philippines, Malaysia and Brunei make competing and overlapping claims in a tangled yet high-stakes rivalry.

The territorial disputes stretch back decades, but took a new twist recently. In an Aug. 3 statement, the State Department criticized China for aggressive actions to reinforce its claims. The next day, China’s Foreign Ministry summoned an American diplomat for a formal protest and announced that the United States “showed total disregard of facts, confounded right and wrong, and sent a seriously wrong message.”

Why this matters is that the United States has announced a pivot toward Asia, a seminal move to counter China’s rising influence, including a rebalancing of forces over the next eight years toward a goal of 60 percent of the Navy in the Pacific, up from half at present. According to Defense Secretary Leon Panetta, that will include six aircraft carriers and a majority of cruisers, destroyers, littoral combat ships and submarines.

The United States is neutral on the territorial claims in the South China Sea. But the State Department’s statement was intended to push back against China’s recent harrying of the Philippines and Vietnam over disputed fishing and oil drilling rights. China has announced that it is upgrading the administrative level of Sansha city, on one of the Paracel islands, to a prefecture and establishing a military garrison there, a further signal of its intent. Worried neighbors are welcoming more port calls from U.S. naval forces.

The U.S. statement called for resolving disputes peacefully. China saw it, quite accurately, as a challenge on behalf of the weaker states in the region and insisted the United States “respect China’s sovereignty and territorial integrity.” What exactly does that entail? China has a very expansive claim to the sea, based on nine dashed lines sketched in a very imprecise fashion on a map six decades ago. The claim encroaches on some of the 200-nautical-mile exclusive economic zones granted to other countries by the United Nations Convention on the Law of the Sea.

China has insisted that it will work out the disputes one by one, and the United States should stay away. But the State Department’s statement accurately asserted that the United States has a “national interest” in the region: not territorial, but to protect regional stability and the huge volume of international shipping that passes through the sea. The sea is clearly a flashpoint. Everyone needs to make sure it does not become a sea of hostility.