Territorial disputes in the South China Sea
Greg Poling, a director at the Center for Strategic and International Studies in Washington, says the U.S. operation was aimed at testing control of the seas, not sovereignty over the disputed islands, to force a clarification of China's claims. Under maritime law, artificial islands were not usually afforded the 12-mile territorial zone, and that the U.S. Navy deliberately chose to send the destroyer near Subi Reef for this reason.
Sources: CNN