An April satellite image of part of the Spratly Islands in the South China Sea. (CSIS Asia Maritime Transparency Initiative/DigitalGlobe/Agence France-Presse via Getty Images)

THE PENTAGON has laid down a fresh marker against China’s worrisome land reclamation effort in the South China Sea. In a congressionally mandated report published last week, the Defense Department outlined a strategy for maritime security in the Asia-Pacific that attempts to counter China’s provocative behavior. The real tests will come later, but they are clearly coming.

In the spring, Adm. Harry B. Harris Jr., commander of the U.S. Pacific Fleet, expressed alarm that China was building artificial land by pumping sand onto live coral reefs and paving over it with concrete. Adm. Harris said China had created about 900 acres of artificial landmass, a “great wall of sand.”

Now the Pentagon has provided stark new details. Since land reclamation got underway in December 2013, China has expanded seven of its eight outposts in the Spratly Islands, and as of this June, had reclaimed “more than 2,900 acres of land.” Other states including Vietnam, Malaysia, the Philippines and Taiwan have done this, too, but at a fraction of the scale. “China has now reclaimed 17 times more land in 20 months than the other claimants combined over the past 40 years,” the report declared.

If these islands are subsequently turned into military outposts with port facilities and airstrips, China will have solidified its sea claims, a serious challenge to the United States, which has pledged to protect freedom of the seas. China’s historic claim to most of the South China Sea is based on a “nine-dashed line” that was drawn on a map decades ago. The island reclamation is putting muscle into that map, allowing China “to establish a more robust power projection presence into the South China Sea,” the report warned. China insists that it is building on its “own islands and reefs” and that it should not concern anyone else.

The Pentagon points out that China has been using its Coast Guard in “a steady progression of small, incremental steps to increase its effective control over disputed areas and avoid escalation to military conflict.” Meanwhile, China’s naval vessels wait over the horizon. Land reclamation cannot be concealed, however; commercial satellite images posted by the Asia Maritime Transparency Initiative at the Center for Strategic and International Studies have documented it.

The United States is prudently shifting military assets toward the Asia-Pacific. Over the next five years, a 30 percent increase is planned in ships assigned to the Pacific Fleet outside the United States, and by 2020, some 60 percent of U.S. naval and overseas air assets will be home-ported in the region.

But 30 percent of the world’s maritime trade goes through the South China Sea each year; other disputed waters in the region are rich in fisheries, oil and gas resources. A conflict could have devastating impact on global trade and commerce. China’s massive exports could suffer terribly. The Pentagon report is a warning to China to tread carefully. The big question is whether such warnings are sufficient to give China’s leaders second thoughts about the great wall of sand.