Almost without notice something important happened a short time ago in the South China Sea. Secretary of State Mike Pompeo publicly announced that “any armed attack on Philippine forces, aircraft or public vessels in the South China Sea will trigger mutual defense obligations under Article IV of our mutual defense treaty.”
President Donald Trump is often accused of abandoning or weakening alliances, and here his administration took an important step affirming an alliance and erasing doubt about America’s commitments. The treaty was somewhat vague because it referred to any attack on the Philippines in the Pacific, and confusion remained as to whether the South China Sea is included in the Pacific. But after resolving that confusion this will strengthen the hand of the Philippines and make them more likely to resist the Chinese attempts to unilaterally build up islands, install military installations there, and then enforce territorial claims.
This is a reverse Dean Acheson moment. In spring of 1950 he gave a speech outlining American commitments in East Asia. From North to South he described the First Island Chain starting in Japan and ending in the Philippines. But he left out South Korea. Many historians say this led Stalin to greenlight the North Korean invasion of South Korea which led to American involvement and the Korean War.
This is the opposite, in that proclaiming the South China Sea as an area in which Chinese aggression against the Philippines will provoke war with America would act as a deterrent. If the Chinese want to use force in the area, such as attacking merchant marine boats or attacking ships that move within 12 nautical miles of their self-claimed territory, they will have to take U.S. reactions and treaty obligations into account. That is why this was an important Dean Acheson moment in the Pacific and should be noted.