An underreported theme of President Trump’s Asia tour was his attention to a regional flashpoint overshadowed by North Korea: the South China Sea. While Mr. Trump avoided public statements on the issue in China, he laid down important markers in Vietnam and the Philippines.

For five years China has escalated tensions by building military bases on artificial islands. Last year a United Nations tribunal found that China’s claim to territorial waters violated international law, but Beijing dismissed the judgment. Chinese vessels...