A satellite image shows what the Asia Maritime Transparency Initiative says appears to bepoint-defense structures for large anti-aircraft guns and close-in weapons systems on an island in the South China Sea. (Digitalglobe/Reuters)

IN THE White House Rose Garden on Sept. 25, 2015, President Xi Jinping stood with President Obama and pledged that “China does not intend to pursue militarization” on outcroppings in the disputed waters of the South China Sea. Other top Chinese officials echoed this promise over the past year, even as airstrips were paved and military exercises carried out offshore.

The Asian Maritime Transparency Initiative of the Center for Strategic and International Studies has just published satellite photography showing that China is, contrary to Mr. Xi’s pledge, militarizing the Paracel and Spratly islands. It has installed point-defense structures for large anti-aircraft guns and close-in weapons systems to defend against cruise missiles. The images reveal identical hexagon-shaped structures constructed this year, which could be supplemented at any time by mobile surface-to-air missile systems if, as expected, the islands are turned into operational air bases.

China claims the weapons are defensive; it described the emplacements as a “slingshot” to ward off a “cocky and swaggering” stranger “at the door of your home.” The message was clearly aimed at the United States. Whether defensive or offensive, the deployments reveal Mr. Xi’s public promises to a U.S. president to be worthless — something that may be more disturbing and dangerous than the arms themselves.

The weapons are just the latest step in China’s drive to take military command of vital waterways that carry more than half the globe’s merchant fleet tonnage. Beijing is thumbing its nose at international law and the rules-based order championed by the United States and its allies. An international tribunal ruled in a case brought by the Philippines that there was “no legal basis” for China’s claim to resources falling within a vast area of the sea defined by an arbitrary “nine dash line.” But the Xi government rejected the decision, disregarded the pledge made at the White House and has moved resolutely down a path of military escalation.

According to a study from the Center for Strategic and Budgetary Assessments (CSBA), China has put installations on four islands in the Paracel group and significant installations on eight islands in the Spratly group. As before, China moved incrementally to avoid triggering a major Western reaction. The Obama administration has taken a few symbolic steps to defend freedom of navigation in the region, but they have had no evident effect on Chinese behavior. On Thursday, China’s navy seized an underwater drone collecting data for a U.S. Navy oceanographic vessel.

China may be taking advantage of the transition to President-elect Donald Trump. We don’t yet know Mr. Trump’s plans for what he has said should be a more muscular approach to China across all fronts. But a more forceful Western response in the South China Sea is overdue. Freedom of navigation and flight exercises could be enlarged. A revised, powerful declaratory policy, a new mechanism for cooperation among the allies, and stationing more U.S. forces and equipment around the region, as recommended in the recent CSBA study, are all worth careful consideration. The United States and its allies must insist on peaceful resolution of the territorial disputes — and judge Mr. Xi by actions rather than unreliable words.