The islands where China had placed anti-aircraft missiles are not at Spartly. They put HQ-9 missiles on Woody island which is at Paracel islands. The nearest country is China, and the only other claimants are Vietnam and Republic of China (Taiwan).
As for Spartly islands, yes, China is building some air strips there. But they are the last one to do so. All the other countries who claim them (Vietnam, Philippines, Malaysia, Taiwan) already have airstrips there for many years. China is the last one to build airstrip there, although it will be the longest once completed.
The US government claims to be neutral in the dispute. Yet US naval ships and aircraft only went to challenge China occupied islands/reefs, while both Vietnamese and Philippines are occupying way more island bases in the same area. There had been a dispute there for many decades with only two skirmish between Vietnam and China back in the 1960s and 1980s. There will be no war if the US military can stop taking sides while claiming otherwise... I very much doubt the US government really want a direct conflict against China. Although no doubt the military industry complex is hoping otherwise... either a direct or indirect (sell more weapons to the countries in disputes) conflict.