You’re confusing foreign policy and domestic policy. Military actions are not domestic - this alliance is to maintain peace and restrain China’s unchecked expansionist goals. Right now, WE are the only thing holding them back.
Federalism. Remember our form of government??? National law enforcement is supposed to handle domestic terrorism.
The Philippines has almost no military, but they have wonderful naval facilities that we built - base the alliance at Subic Bay, thereby keeping that nation safe. This helps fence the PLAN into the South China Sea.
Without the Philippines in place, China is into the Pacific easily.
I’m not sure about the Philippines. If we get too re-invested in Subic Bay then Duterte is liable to threaten to tilt toward China unless we pay up, regardless of how much we might have already promised/invested. I’m just not sure he can be trusted. It’s just such a shame that Subic Bay is so perfectly positioned for the current situation, but is controlled by someone so untrustworthy.
Vietnam is an interesting potential partner also. It’s government might be more likely to keep its word, but can we really have a mutual defense pact with a communist state?
The rest of the quad plus South Korea and New Zealand are both democracies and reliable, but less well positioned for conflict in the South China Sea,