The current territorial boundaries of the Republic of the Philippines were legally established on July 4, 1946 when the U.S. relinquished certain territories it "bought" from Spain in 1898 through the Treaty of Paris. The U.S. did not receive any compensation for these territories from the Republic of the Philippines. These territories were essentially "donated" to the Republic of the Philippines.
Currently, there is an on-going effort by the People's Republic of China to physically grab certain portions of these territories as "her own". These are the western territories of the Republic of the Philippines bordering the South China Sea.
The People's Republic of China was only established on October 5, 1949.
Due to the overwhelming advantage in terms of military power of the People's Republic of China, it appears that the Republic of the Philippines may not be in a position to physically resist the People's Republic of China from physically occupying these territories.
Once the Donee, Republic of the Philippines, involuntarily relinquishes control over these donated territories, such territories should be deemed as abandoned, and the Donor, the United States of America, shall have the primary legal right to regain possession of her donation. The U.S. donated these territories to the Republic of the Philippines, and not the People's Republic of China.
Any territory which the Republic of the Philippines loses control of, therefore, shall revert back to its former U.S. Territory status, which should be defended with any means the U.S. has at its disposal as mandated by the U.S. Constitution.