China’s decision to station surface-to-air missiles on a disputed island in the South China Sea is the latest in a series of provocative acts that is fueling regional tensions. This unwise move raises new doubts about President Xi Jinping’s pledge not to “pursue militarization” in a vital waterway and passage for $5 trillion in annual trade.

The two HQ-9 missile batteries were deployed recently on Woody Island in the Paracel chain. The missiles reportedly have a range of about 125 miles and are capable of destroying aircraft, cruise missiles and ballistic missiles. In theory, the weapons could have a legitimate purpose, enabling China to better defend its naval bases on Hainan Island, which is 273 miles away. Although also claimed by Taiwan and Vietnam, the Paracels have been under Beijing’s control for four decades, and Chinese forces have been stationed there for years.

But the timing of the deployment and the way in which it was done makes it impossible to blindly accept a self-defense rationale. It is part of a pattern in which China in recent years has claimed “indisputable sovereignty” over 90 percent of the South China Sea and asserted jurisdiction not just over the Paracels, a recognized island chain, but also many obscure reefs and rocks in the Spratly Islands.

Beijing’s purpose is to box out rival claims from other countries, including Vietnam, the Philippines, Indonesia and Malaysia. As part of this effort, it has also turned seven of those specks into more substantial islets, some big enough to hold military bases, while claiming jurisdiction of the waters around them.