Why? Perhaps to engender a military incident that would boost his popularity around election—you know, “rally ‘round the flag, boys” and all that.
As a foreigner, I am a bit dismayed at the degree of nay-saying that I see sometimes on FR. This is a good move for American Geo-Political strategy and for the smaller countries in the region and don’t go off on a tangent about Iraq and Afghanistan and all that. On those points, I agree. Hillary in a rare show of sense, got it spot on that Asia is where the future will be decided, not Afghanistan. Australia is the safest bet for a future military base in the region without and in your face presence in Vietnam (if that were even feasible) or a return to Philippines.
This move sends the Chinese a message and it serves that purpose well. They will be afraid to be as belligerent as they have been and will in turn serve American interests by keeping China pegged in to the little water strip skirting their coast instead of claiming the whole South China sea.