I'd say the US is only partially sensitive to those islands in the south China sea in how they impact maritime navigation for their navy. Key to understanding t

I'd say the US is only partially sensitive to those islands in the south China sea in how they impact maritime navigation for their navy. Key to understanding the SCS dispute is that in comes in two parts. One is who the islands, the actual land, belongs to. Second is how the land impacts maritime rights over the surrounding waters in terms of control over navigation and natural resources. The US has taken no explicit position on the land and only defends a subset of maritime rights useful to itself. It has supported none of the land claims of the other parties, hasn't pledged support to defend the islands of the Philippines, arguably it's closest historical ally in the dispute, it hasn't done anything to stop China's island construction or seize them before they were fortified, and it hasn't done anything to protect local fishermen from being pushed out of fishing grounds by the Chinese coast guard. The only thing the US has done is maintain its right to sail its navy wherever it chooses and dare all the countries in the region, not just China, to stop them. Arguably these half-hearted exercises and the US's refusal to take a side in the territory dispute is why a lot of other countries in the region are lukewarm about aggressively staking their claims. The US is fighting for a narrow set of interests that benefits itself but will not fight for the interests of other claimants that have no direct benefits to the US Navy. If the Philippines decides to build an airstrip on one of their islands and China starts intercepting their construction crews or even goes as far as amphibious landings the Philippines has little faith that the US won't just frame this as a problem they instigated themselves.