STEP BY incremental step, China appears to be digging into disputed territories in the South China Sea. That is the inescapable conclusion from the latest report that it has stationed a modern surface-to-air weapons system, the HQ-9, on Woody Island, the largest in the Paracel chain, controlled by China but claimed by Vietnam and Taiwan. Though the United States has no territorial claim, the new Chinese installations seriously threaten a central goal of U.S. policy in the region: to sail, fly and operate anywhere that international law allows. The installations also threaten to impose China’s unilateral resolution on claims that the United States has urged be settled through negotiation.

It is no accident that China carried out this move as President Obama hosted allies from Southeast Asia at a summit in California on Feb. 15 and 16. But this is more than symbolism. Overhead satellite photography shows that China has deployed two batteries of four launchers each to the island. The missiles have an effective range of some 62 miles and would be useful if China decided at some point to declare an air defense identification zone, asserting control over air traffic. China has built up Woody Island to a greater extent than the Spratly Islands, also disputed, where it has been filling in remote outcroppings and laying down airfields. China seems to think the best way to carry out this expansion is through small actions, avoiding a confrontation with the United States. But over time, small actions add up. If the surface-to-air missiles are visible on one island, how long before they show up on another?

Last autumn, in the White House Rose Garden, Chinese President Xi Jinping pledged to Mr. Obama that, when it came to construction activity in the Spratly Islands, China does not “target or impact any country, and China does not intend to pursue militarization.” It is true that Mr. Xi was not speaking technically of the Paracels. But the latest action calls into question the spirit of his pledge and China’s frequent assurances that its intentions in the South China Sea are peaceful. This is a situation in which deeds speak much louder than words, and the deeds show a steady expansion of China’s maritime claims. Some have called it the Great Wall of sand.

Secretary of State John F. Kerry reacted to China’s latest deployments by saying, “There is every evidence, every day that there has been an increase of militarization of one kind or another. It’s of serious concern.” Mr. Kerry promised to have more “conversations” with the Chinese. Conversation is fine, but at this point, insufficient. The United States has twice in recent months carried out freedom-of-navigation exercises in these waters; now, there should be progressively more challenging maneuvers. It is important to get the signals right. Failure to act would be exactly the wrong signal in the face of China’s ever-enlarging garrisons in the sea.