U.S. help to the Philippines would trigger Chinese 3.3-megaton DF-3A IRBMs
I want to make an important point regarding U.S. refusal to help the Philippines. Strategically speaking, the Philippines is safer without U.S. help.
As soon as an U.S. warship assists the Philippines, China's pledge of not using thermonuclear weapons is null and void. The U.S. is a nuclear weapons state. The participation of a nuclear weapons state authorizes the use of Chinese 3.3-megaton DF-3A IRBM thermonuclear weapons against the Philippines.
In sum, the U.S. is trying to prevent China's Second Artillery from coming into play. There is also a massive Chinese naval base on Hainan Island with Type 094 Jin-class submarines. Each Type 094 Jin carries 12 JL-2 SLBMs with 3-MIRVs per SLBM.
China's DF-3A/CSS-2 IRBM with 3.3 megaton warhead
Each Chinese Type 094 Jin-class submarine carries twelve JL-2 SLBMs. Each JL-2 SLBM has three MIRVs. A Type 094 Jin carries a total of 36 thermonuclear warheads. China has at least 4 Type 094 Jins for a total of 144 thermonuclear warheads to be deployed against the Philippines.