Fire and maneuver in the Pacific Ocean requires submarines, ships, airplanes and other non-fixed platforms for projectile, bomb, and missile delivery.
In World War II, the Japanese were able to seize and occupy islands with small defense forces - Wake Island, Attu, and Kiska, and eventually the Philippines.
Islands are fixed fortifications that can be over-run because of their inherent lack of mobility.
For the Chinese Communist Party, trade, diplomacy, espionage, and political infultration are their most effective offensive weapons. They won't need to "invade" if their host(s) "invite" them in.
There is strategic value for the air force, navy, army, coast guard, and NOAA to build or occupy islands to check the PLA's expansion, but, they will be indefensible without naval submarine, surface, and air superiority.
Marines are supposed to advance, not be stationary.