OVER THE SOUTH CHINA SEA—China has fully militarized at least three of several islands it built in the disputed South China Sea, arming them with anti-ship and anti-aircraft missile systems, laser and jamming equipment, and fighter jets in an increasingly aggressive move that threatens all nations operating nearby, a top U.S. military commander said Sunday. U.S. Indo-Pacific commander Adm. John C. Aquilino said the hostile actions were in stark contrast to Chinese leader Xi Jinping’s past assurances that Beijing would not transform the artificial islands in contested waters into military bases. The efforts were part of the Chinese communist regime’s flexing its military muscle, he said. “I think over the past 20 years we’ve witnessed the largest military buildup since World War II by the PRC,” Aquilino told The Associated Press in an interview, using the initials of China’s formal name. “They have advanced all their capabilities and that buildup …