What would happen to the Philippines if a naval conflict broke out with China?
A blockade of China would be conducted from the Indian ocean and the final line would be at the Malay barrier bounded by the Lombok, Sunda and Malacca Straits. That would be outside the range of Chinese area denial systems.
The strategic imperative to maintain lines of communication with Japan, Korea, Australia and optionally Taiwan can be met by convoying the Luzon strait bound ships south and east of the Malay barrier, either to the north or possibly completely around Australia. Given the importance of these allies, the USN will do everything necessary to see Japan et al are supplied.
The USN is probably not, nor ever was prepared to fight a major action in the South China Sea littoral. Not a single major WW2 fleet engagement occured in the SCS. All occurred EAST of the Philippines, on the Pacific deep water side. The SCS is more Australia’s war patrol station and its fleet of diesel electric subs is optimized for littoral operations, though that is changing now with USN’s Virginia class subs and AUUVs.
To defend the Philippines it must be supplied. That means convoying ships to the RP’s west coast ports (Manila, Batangas, Subic) since it has no east coast ports on the Pacific side. That would put the USN precisely where it doesn’t want to be, under the missiles of China’s PLAN and Second Artillery Corps. Not only that, PLAN has an immense stock of mines. China has in fact the biggest mine force in the world. Japan, not coincidentally, has the biggest minesweeping force, though USN is ramping up.
The Visayan and Mindanao ports may be fitfully supplied by coastal shipping. But Luzon will starve without convoys. Manila in particular will suffer. The most likely escorts of food and fuel and medicine, if there are any convoys at all, will come from Japan. It won’t be out of altruism. Luzon is vital to the security of Taiwan and the Ryukyus. The Philippines is a “nice to have” for the US but it is a must have for Japan.
That’s why the Japanese fleet died off eastern Philippines; why the Japanese Army died in Luzon. But the defense of the Philippines was always debated, as far back as War Plan Orange in Washington. In 1944 Roosevelt was still undecided over whether to bypass the Philippines until MacArthur convinced him that it was needed to interdict the China coast and prevent the Kwangtung Army from reinforcing the Japanese home islands.
The Philippines is the victim of geography, a weak and corrupt state surrounded by India, China, Japan, Russia, Korea and the United States — the most powerful countries in the world. Heaven watch the Philippines because nobody else will.