Prior to 1930 (1946), actually, the U.S. had claims in this area, through its possession of the Philippines. This is because the Philippines are one of 10 so-called ASEAN (Southeast Asian) nations. Even to this day, the U.S. has certain treaty rights in the Philippines. That is to say that the U.S. retains a "proprietary" interest in Philippine affairs, even though she has granted independence to the Philippines.
Other formal claimants in the general area before 1945 were the French (Indochina), the Dutch (East Indies, modern Indonesia), and the British (Malaysia, Singapore, Brunei). These three powers are gone, the U.S. is not.
The ASEAN nations basically surround this waterwawy shown in the map. It is the "fourth" great region of Asia (after China, India, and Japan). If they were a country, they would have 500 million people and over 1 million square miles, giving them great economic and strategic importance. And while ASEAN is less rich than the other countries, the waterway affects ASEAN much more; that is, all of ASEAN versus e.g. only part of China.